Avoid unwanted "null"s in server capabilities

[dimbleby]

Description (e.g. "Related to ...", etc.)

Avoid setting fields in server capabilities to None, because explicitly setting them causes pydantic to serialize them, which is not what is wanted.

See pappasam/jedi-language-server#108

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly
• CONTRIBUTORS.md was updated, as appropriate
• Changelog has been updated, as needed (see CHANGELOG.md)

[danixeee]

Looks good.