Use empty string instead of None for fallback. Fixes #2313.

diff --git a/spyderlib/utils/introspection/plugin_manager.py b/spyderlib/utils/introspection/plugin_manager.py
index 9685848..0db27dd 100644
--- a/spyderlib/utils/introspection/plugin_manager.py
+++ b/spyderlib/utils/introspection/plugin_manager.py
@@ -158,7 +158,7 @@ class CodeInfo(object):
                     break
                 position -= 1
         else:
-            self.docstring = None
+            self.docstring = ''

         self.position = position

@@ -167,8 +167,8 @@ class CodeInfo(object):
             self.column = 0
             self.line_num = 0
             self.line = ''
-            self.obj = None
-            self.full_obj = None
+            self.obj = ''
+            self.full_obj = ''
         else:
             self._get_info()

@@ -229,6 +229,7 @@ class CodeInfo(object):
             right += 1
         if left and right < len(self.source_code):
             return self.source_code[left: right]
+        return ''

     def __eq__(self, other):
         try:

spyderlib/utils/introspection/plugin_manager.py

@@ -158,7 +158,7 @@ def __init__(self, name, source_code, position, filename=None,
                     break
                 position -= 1
         else:
-            self.docstring = None
+            self.docstring = ''
 
         self.position = position
 
@@ -167,8 +167,8 @@ def __init__(self, name, source_code, position, filename=None,
             self.column = 0
             self.line_num = 0
             self.line = ''
-            self.obj = None
-            self.full_obj = None
+            self.obj = ''
+            self.full_obj = ''
         else:
             self._get_info()
 
@@ -229,6 +229,7 @@ def _get_docstring(self):
             right += 1
         if left and right < len(self.source_code):
             return self.source_code[left: right]
+        return ''
 
     def __eq__(self, other):
         try: