fix(ngTransclude): only compile fallback content if necessary

If the instance of the directive does provide transcluded content, then the fallback
content should not be compiled and linked as it will never be used.

If the instance of the directive does not provide transcluded content, then the
transcluded scope that was created for this non-existent content is never used,
so it should be destroy in order to clean up unwanted memory use and digests.

Fixes #14768
Fixes #14765
Closes #14775