Fix: python serialization edge case

[georgesittas]

This PR addresses a bug in the Python code serialization logic. When walking objects and looking up function globals, we only add them to the environment if the fetched objects are not falsey, which leads to excluding falsey primitives such as 0, False and None. A minimally-reproducible example for this is shown below:

import pandas as pd
from sqlmesh import model

Z = 0

@model(name="foo", columns={"id": "int"})
def entrypoint(context, *args, **kwargs):
    return pd.DataFrame({"id": [Z]})

# Failed models
# 
#  "foo"
#
#    File 'models/test.py' (or imported file), line 2, in entrypoint
#        def entrypoint(context, *args, **kwargs):
#            return pd.DataFrame({'id': [Z]})
#          NameError: name 'Z' is not defined
#
# Error: Plan application failed.

No additional context here. I was simply playing around locally and came across this issue, so figured I'd fix it.