ignore None return value from get_generators signal

[avaris]

Resolves: #2834

Ignores if nothing was returned from get_generators signal. Some plugins were using this signal primarily because it provided a convenient place in the run timeline and did not return any generator to register. They were cluttering log with error messages even though everything was working as expected.

• [+] Ensured tests pass and (if applicable) updated functional test output
• [+] Conformed to code style guidelines by running appropriate linting tools

[justinmayer]

Looks good to me. Many thanks for fixing this.

Once this and the Pygments 2.8-related PR are merged, we can issue a new release containing both changes (and any other minor commits since the release last month).