Update libgap.pyx

now returning NULL for undefined GAP variables, instead of raising GapError

Author: fchapoton

src/sage/libs/gap/libgap.pyx

@@ -479,9 +479,7 @@ class Gap(Parent):
             1
             sage: libgap.unset_global('FooBar')
             sage: libgap.get_global('FooBar')
-            Traceback (most recent call last):
-            ...
-            GAPError: Error, VAL_GVAR: No value bound to FooBar
+            NULL
         """
         is_bound = self.function_factory('IsBoundGlobal')
         bind_global = self.function_factory('BindGlobal')
@@ -504,9 +502,7 @@ class Gap(Parent):
             1
             sage: libgap.unset_global('FooBar')
             sage: libgap.get_global('FooBar')
-            Traceback (most recent call last):
-            ...
-            GAPError: Error, VAL_GVAR: No value bound to FooBar
+            NULL
         """
         is_readonlyglobal = self.function_factory('IsReadOnlyGlobal')
         make_readwrite = self.function_factory('MakeReadWriteGlobal')
@@ -536,9 +532,7 @@ class Gap(Parent):
             1
             sage: libgap.unset_global('FooBar')
             sage: libgap.get_global('FooBar')
-            Traceback (most recent call last):
-            ...
-            GAPError: Error, VAL_GVAR: No value bound to FooBar
+            NULL
         """
         return make_any_gap_element(self, GAP_ValueGlobalVariable(str_to_bytes(variable)))