Fix ec2 filter by empty tag value (#3605)

* Fix ec2 filter by empty tag value

Return `None` instead of an empty string when the tag key does not exist
and replace the falsy check with a more explicit `is None`, which allows
empty string values to correctly pass through the filter comparator.

Behavior confirmed against a real AWS backend.

Closes #3603

* Make test case more explicit

Test case now pulled directly from the issue report (#3603).

Co-authored-by: Bert Blommers <bblommers@users.noreply.github.com>

moto/ec2/models.py

@@ -249,7 +249,7 @@ class TaggedEC2Resource(BaseModel):
                 if tag["key"] == tagname:
                     return tag["value"]
 
-            return ""
+            return None
         elif filter_name == "tag-key":
             return [tag["key"] for tag in tags]
         elif filter_name == "tag-value":

moto/ec2/utils.py

@@ -458,7 +458,7 @@ def filter_internet_gateways(igws, filter_dict):
 def is_filter_matching(obj, filter, filter_value):
     value = obj.get_filter_value(filter)
 
-    if not filter_value:
+    if filter_value is None:
         return False
 
     if isinstance(value, six.string_types):