Don't try to uncompress if there's nothing to uncompress

For example, when upgrading a database from an older schema, some
fields which were meant to be decompressed would actually be None
because they have no values.
If the value is None, just return None.

Change-Id: I2802b1aabf3be5aba6b066c85b5d13abaef53ca0

ara/models.py

@@ -123,7 +123,10 @@ class CompressedData(types.TypeDecorator):
         return zlib.compress(json.dumps(value))
 
     def process_result_value(self, value, dialect):
-        return json.loads(zlib.decompress(value))
+        if value is not None:
+            return json.loads(zlib.decompress(value))
+        else:
+            return value
 
     def copy(self, **kwargs):
         return CompressedData(self.impl.length)
@@ -143,7 +146,10 @@ class CompressedText(types.TypeDecorator):
         return zlib.compress(value if value else '')
 
     def process_result_value(self, value, dialect):
-        return zlib.decompress(value)
+        if value is not None:
+            return zlib.decompress(value)
+        else:
+            return value
 
     def copy(self, **kwargs):
         return CompressedText(self.impl.length)