Bugfix: Raw LLM response: null -> True

[thekaranacharya]

Add fix for #479

• Remove super().verify() method, clean up the code and convert to a normal method which will be used by all the inherited classes
• New method: is_optional_and_null() now returns True if field is optional and null, else False
• Gets called in all child classes' verify() method -> if True, will return None rather than simply returning the boolean itself.

Other updates to code:

• Add bug-fixes in OpenAI v1 implementation
  • Correctly parse OpenAI output - either using content or function_call arguments
• Other minor bug-fixes

[notion-workspace]

OpenAI output null values for keys are converted to True in validated response