frontend: Fix crash on startup when using tbar apis in non-studio-mode

[psyirius]

Description

Fixes crash on accessing tbar apis such as obs_frontend_get_tbar_position, obs_frontend_set_tbar_position and obs_frontend_release_tbar when not in studio-mode on obs startup.

Motivation and Context

TBar apis should be a noop if studio-mode is active.
Instead it crashes obs if accessed in non-studio mode.

How Has This Been Tested?

Built OBS with this pr and accessed the tbar apis in non-studio mode from a plugin.
Everything working fine.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.