Ensure no empty strings are fed to TranslatableMarkup

Follow-up to #2570285: Make sure TranslatableMarkup accepts string values only

Follow-up to #2557113: Make t() return a TranslationWrapper object to remove reliance on a static, unpredictable safe list

Problem/Motivation

Empty strings are not valid input for the TranslatableMarkup constructor.

Proposed solution

Fail when input doesn't make sense. Test for empty strings before creating new TranslatableMarkup objects, and leave them as plain strings if they are empty.