Remove parameter value/return nullability in IImageSourceParser, ILocalLinkParser and IMacroParser

[AndyButland]

This update just aligns the nullability of the parameter and return value of the three interfaces that define parsers for rich text content. It's not essential, but nice to tidy up.

Implemented in response to this comment.

There is no implementation of this class in the CMS itself.

[ronaldbarendse]

Since we don't need to parse the value if it's null, we can avoid some methods calls by removing the nullability from the value parameter and return type (all implementations would otherwise do a if (value is null) return null; check anyway).