fix(voip-mumble): nullptr check

[tens0rfl0w]

Goal of this PR

This fixes a nullptr exception when trying to write to the Audio Processing Module while the audioCaptureClient is invalid.

We now also apply the last voice intent mode to the new audio device instead of the default ('speech') mode.

How is this PR achieving the goal

Added a nullptr check before trying to write to the Audio Processing Module.

This PR applies to the following area(s)

FiveM

Successfully tested on

Game builds: Not applicable

Platforms: Windows (Client)

Checklist

• Code compiles and has been tested successfully.
• Code explains itself well and/or is documented.
• My commit message explains what the changes do and what they are for.
• No extra compilation warnings are added by these changes.

Fixes issues

Reported here: https://discord.com/channels/779705925577080842/1301532596240515082 (Cfx.re Engineering Group)