Try to fix test_gdb on s390x SLES 3.x

Ignore empty lines in stderr.
diff --git a/Lib/test/test_gdb.py b/Lib/test/test_gdb.py
index 8e3ccb0..ccef422 100644
--- a/Lib/test/test_gdb.py
+++ b/Lib/test/test_gdb.py
@@ -208,6 +208,8 @@
             'warning: ',
             )
         for line in errlines:
+            if not line:
+                continue
             if not line.startswith(ignore_patterns):
                 unexpected_errlines.append(line)