Allow strictNullHandling to work when using a custom filter

[Neaox]

Fixes #278

[ljharb]

Can you please provide a test case, so it's clear what you expected to work, and how it failed?

[Neaox]

Added a test as requested.

An explanation of the issue:

If you specify a custom filter in your stringify options the if case here (https://github.com/ljharb/qs/pull/279/files/ab07ce71f33704ecfac7298b055be8f481edf277#diff-dd5072864997b356b1727023f837c921L48) is run. Due the statement on line 52 (https://github.com/ljharb/qs/pull/279/files/ab07ce71f33704ecfac7298b055be8f481edf277#diff-dd5072864997b356b1727023f837c921L52) being an else if, instead of an if the null handling is not run, which then causes an error when attemping to get the keys of a null object.

By changing this else if to an if the issue is fixed.

[ljharb]

Thanks; I also added a test case for a serializeDate that returns null, which your fix also addresses.