Do not set __doc__ attribute if there is no doc string (enables test_docstring)

Author: steve-s

graalpython/com.oracle.graal.python.test/src/tests/unittest_tags/test_fstring.txt

@@ -1,4 +1,5 @@
 *graalpython.lib-python.3.test.test_fstring.TestCase.test__format__lookup
+*graalpython.lib-python.3.test.test_fstring.TestCase.test_docstring
 *graalpython.lib-python.3.test.test_fstring.TestCase.test_assignment
 *graalpython.lib-python.3.test.test_fstring.TestCase.test_ast_compile_time_concat
 *graalpython.lib-python.3.test.test_fstring.TestCase.test_backslash_char

graalpython/com.oracle.graal.python.test/testData/goldenFiles/AssignmentTests/annotationType02.tast

@@ -14,7 +14,7 @@ ModuleRootNode Name: <module 'annotationType02'> SourceSection: [0,28]`def fn():
           FunctionDefinitionNode Name: fn SourceSection: None
             Arguments: None
             KwArguments: None
-            Documentation: StringLiteralNode: Empty
+            Documentation: None
             FreeVarSlots: None
             ExecutionSlots:
               FreeVarsSlots: None

graalpython/com.oracle.graal.python.test/testData/goldenFiles/AssignmentTests/assignment06.tast

@@ -14,7 +14,7 @@ ModuleRootNode Name: <module 'assignment06'> SourceSection: [0,29]`def fn():↵
           FunctionDefinitionNode Name: fn SourceSection: None
             Arguments: None
             KwArguments: None
-            Documentation: StringLiteralNode: Empty
+            Documentation: None
             FreeVarSlots: None
             ExecutionSlots:
               FreeVarsSlots: None

graalpython/com.oracle.graal.python.test/testData/goldenFiles/AssignmentTests/assignment07.tast

@@ -14,7 +14,7 @@ ModuleRootNode Name: <module 'assignment07'> SourceSection: [0,29]`def fn():↵
           FunctionDefinitionNode Name: fn SourceSection: None
             Arguments: None
             KwArguments: None
-            Documentation: StringLiteralNode: Empty
+            Documentation: None
             FreeVarSlots: None
             ExecutionSlots:
               FreeVarsSlots: None

graalpython/com.oracle.graal.python.test/testData/goldenFiles/AssignmentTests/augassign13.tast

@@ -14,7 +14,7 @@ ModuleRootNode Name: <module 'augassign13'> SourceSection: [0,17]`def fn (): x +
           FunctionDefinitionNode Name: fn SourceSection: None
             Arguments: None
             KwArguments: None
-            Documentation: StringLiteralNode: Empty
+            Documentation: None
             FreeVarSlots: None
             ExecutionSlots:
               FreeVarsSlots: None

graalpython/com.oracle.graal.python.test/testData/goldenFiles/AssignmentTests/augassign14.tast

@@ -14,7 +14,7 @@ ModuleRootNode Name: <module 'augassign14'> SourceSection: [0,61]`def _method(*a
           FunctionDefinitionNode Name: _method SourceSection: None
             Arguments: None
             KwArguments: None
-            Documentation: StringLiteralNode: Empty
+            Documentation: None
             FreeVarSlots: None
             ExecutionSlots:
               FreeVarsSlots: None

graalpython/com.oracle.graal.python.test/testData/goldenFiles/AwaitAndAsyncTests/asyncFor01.tast

@@ -14,7 +14,7 @@ ModuleRootNode Name: <module 'asyncFor01'> SourceSection: [0,39]`async def f():
           FunctionDefinitionNode Name: f SourceSection: None
             Arguments: None
             KwArguments: None
-            Documentation: StringLiteralNode: Empty
+            Documentation: None
             FreeVarSlots: None
             ExecutionSlots:
               FreeVarsSlots: None

graalpython/com.oracle.graal.python.test/testData/goldenFiles/AwaitAndAsyncTests/asyncFor02.tast

@@ -14,7 +14,7 @@ ModuleRootNode Name: <module 'asyncFor02'> SourceSection: [0,42]`async def f():
           FunctionDefinitionNode Name: f SourceSection: None
             Arguments: None
             KwArguments: None
-            Documentation: StringLiteralNode: Empty
+            Documentation: None
             FreeVarSlots: None
             ExecutionSlots:
               FreeVarsSlots: None

graalpython/com.oracle.graal.python.test/testData/goldenFiles/AwaitAndAsyncTests/asyncWith01.tast

@@ -14,7 +14,7 @@ ModuleRootNode Name: <module 'asyncWith01'> SourceSection: [0,34]`async def f():
           FunctionDefinitionNode Name: f SourceSection: None
             Arguments: None
             KwArguments: None
-            Documentation: StringLiteralNode: Empty
+            Documentation: None
             FreeVarSlots: None
             ExecutionSlots:
               FreeVarsSlots: None

graalpython/com.oracle.graal.python.test/testData/goldenFiles/AwaitAndAsyncTests/asyncWith02.tast

@@ -14,7 +14,7 @@ ModuleRootNode Name: <module 'asyncWith02'> SourceSection: [0,47]`async def f():
           FunctionDefinitionNode Name: f SourceSection: None
             Arguments: None
             KwArguments: None
-            Documentation: StringLiteralNode: Empty
+            Documentation: None
             FreeVarSlots: None
             ExecutionSlots:
               FreeVarsSlots: None