Implement OptionalFromRequest for Form and RawForm

[avalon1610]

Motivation

This change enables the use of Form as an optional extractor in certain scenarios.

Solution

This PR implements OptionalFromRequest for Form and RawForm. The behavior is as follows:

• For GET or HEAD requests, if there are no query parameters, the extractor will return None. Otherwise, it will parse the form data using the original logic.
• For other HTTP methods, if the Content-Type header is not present, the extractor will return None. Otherwise, it will parse the form data using the original logic.

Additionally, the FromRequest implementation for RawForm does not include a check for the HEAD method. I believe this is a bug, as Form and RawForm should behave consistently. Is this understanding correct?