Date/Time: Make sure `get_the_date()` and related functions return correct...

Date/Time: Make sure `get_the_date()` and related functions return correct time if the format was specified as `false`.

Technically, the `$format` argument should always be a string, but passing `false` used to work before [47808], so this restores backward compatibility.

The list of affected functions:

- `get_the_date()`
- `get_the_time()`
- `get_comment_date()`
- `get_comment_time()`

Props wittich, Rarst, akabarikalpesh, SergeyBiryukov.
Merges [48912] to the 5.5 branch.
Fixes #51184.
Built from https://develop.svn.wordpress.org/branches/5.5@48921


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48683 1a063a9b-81f0-0310-95a4-ce76da25c4cd