Fix null numeric filter conditions

[bsbodden]

Ensures that the Num class correctly handles 0 as a valid value. This PR modifies the __str__ method to check if the value is None explicitly instead of using a condition that treats 0 as false.

Context:

Q: Is there a way possible to set the filter to a num 0. Below is the client query,
filter_conditions: FilterExpression = ((Tag("doc_base_id") == doc_base.id) &
                        (Tag("file_id") == file_id) &
                        (Num("chunk_number") == chunk_num))
When I instantiate the query with a non-zero chunk_number, the generated query looks as expected and the result only contains the requested chunk:
[doc_base.id](http://doc_base.id/)
'AIGuildDemo'
file_id
'e9ffbac9ff6f67cc'
chunk_num
1
str(filter_conditions)
'((@doc_base_id:{AIGuildDemo} @file_id:{e9ffbac9ff6f67cc}) @chunk_number:[1 1])'
However, when I set chunk_num to 0, chunk_number gets left out of the query entirely, and the result of the query is all chunks with that doc_base_id and file_id:
[doc_base.id](http://doc_base.id/)
'AIGuildDemo'
file_id
'e9ffbac9ff6f67cc'
chunk_num
0
str(filter_conditions)
'(@doc_base_id:{AIGuildDemo} @file_id:{e9ffbac9ff6f67cc})'
Based on a look through redisvl documentation, it seems that this issue might be related to this feature where passing “None” will drop the filter
Is there way way to use redisvl to filter entries where chunk_number is 0?

[tylerhutcherson]

Great fix. LGTM