One could never rebuild the cache given the existing conditional logic (premature return to exception based solely on fallback_to_default). Now, if a user disables fallback and enables rebuild, the rebuild logic will execute.

This allows a user to install a font and run:

import matplotlib.font_manager as fm
fm.findfont("Font Spec", fallback_to_default=False, rebuild_if_missing=True)

to force a rebuild of the font cache.

It was a simple fix to the conditional logic: if fallback is False, don't return with error if rebuild is True, but instead set result to empty string (used later in the rebuild code) so that the rebuild code can actually be proceeded to.

PR summary

PR checklist

• [N/A] "closes #0000" is in the body of the PR description to link the related issue

• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• [N/A] Documentation complies with general and docstring guidelines