Fix match groups default argument

Author: msimacek

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

@@ -67,6 +67,7 @@
 *graalpython.lib-python.3.test.test_re.ReTests.test_re_escape_non_ascii_bytes
 *graalpython.lib-python.3.test.test_re.ReTests.test_re_findall
 *graalpython.lib-python.3.test.test_re.ReTests.test_re_groupref_exists
+*graalpython.lib-python.3.test.test_re.ReTests.test_re_match
 *graalpython.lib-python.3.test.test_re.ReTests.test_re_split
 *graalpython.lib-python.3.test.test_re.ReTests.test_re_subn
 *graalpython.lib-python.3.test.test_re.ReTests.test_repeat_minmax_overflow_maxrepeat

graalpython/lib-graalpython/_sre.py

@@ -166,7 +166,7 @@ def group(self, *args):
     def groups(self, default=None):
         lst = []
         for arg in range(1, self.compiled_regex.groupCount):
-            lst.append(self.__group__(arg))
+            lst.append(self.__group__(arg, default))
         return tuple(lst)
 
     def __groupidx__(self, idx):
@@ -175,11 +175,11 @@ def __groupidx__(self, idx):
         else:
             return idx
 
-    def __group__(self, idx):
+    def __group__(self, idx, default=None):
         idxarg = self.__groupidx__(idx)
         start = self.result.getStart(idxarg)
         if start < 0:
-            return None
+            return default
         else:
             return self.input_str[start:self.result.getEnd(idxarg)]