Only resolve theme suggestions when a preprocessor is actually registered...

Only resolve theme suggestions when a preprocessor is actually registered against one. Resolving them invokes every hook_theme_suggestions_HOOK() implementation a second time, and does so with variables that have already been through the preprocess chain. Those implementations are written against the variables core passes before preprocessing, so re-invoking them late can hand them a structure they do not expect and fatal.

Closes #3624397