Make sure $tz is not empty in get_gmt_from_date()

[johnjamesjacoby]

It's possible that $tz might still be empty by default, which causes errors when posting new content.

See r17920 for where this was introduced.

[ryan]

Perhaps we need to revert back and just remove the class_exists() check.

[johnjamesjacoby]

Yep, that's what needs to happen.

My patch prevents the error, but results in the post_date and post_date_gmt being the same.

[ryan]

In [17958]:

Revert part of [17920]. Support for empty tz must remain. Props johnjamesjacoby. fixes #17448