SensorFusion: Do not use the sensor variables if they are not init'ed

The member variables for the optional sensors (mMag & mGyro) could be
used even when they are not initialized, leading to failed fusion result
in corner cases.

Let's check those variables are properly initialized before using
them.

Also add minor fixes for the coding style.

Bug: 425971589
Test: Run CtsSensorTestCases for auto CF (no mag mode)
Flag: EXEMPT bugfix

Change-Id: Iecd215d585fdab62bb2ff895c4b78694abd70a28