Enforce False case for flow run id is null

[zangell44]

This PR fixes a bug @znicholasbrown found when trying to filter only for tasks that are contained within a flow run.

See also https://github.com/PrefectHQ/nebula/pull/7809.

Example

Checklist

• This pull request references any related issue by including "closes <link to issue>"
  • If no issue exists and your change is not a small fix, please create an issue first.
• If this pull request adds new functionality, it includes unit tests that cover the changes
• This pull request includes a label categorizing the change e.g. maintenance, fix, feature, enhancement, docs.

For documentation changes:

• This pull request includes redirect settings in netlify.toml for files that are removed or renamed.

For new functions or classes in the Python SDK:

• This pull request includes helpful docstrings.
• If a new Python file was added, this pull request contains a stub page in the Python SDK docs and an entry in mkdocs.yml navigation.

[chrisguidry]

I might like to see the regression test for this