fix(files_sharing): Restore password guard return for new public shares

Creating a new public share without a password silently succeeded: the
password error was shown but execution continued, and the share was
created without a password. Users had to save a second time for the
password to apply.

The password guard now blocks the save when a new public share is
missing a password. Non-public shares (user/group) are never blocked
by this guard, so they remain unaffected.

Tests invoke the real saveShare method against a stubbed context and
cover the save-twice symptom and the non-public-share regression.

Author: nfebe