[3.x] Fix zero scale in particle shader

[lawnjelly]

Fixes the lower bounding of scale when given zero input.

The previous bug was due to sign returning 0 with 0.0 input, rather than 1.

Fixes #80863

Notes

• Regression introduced in [3.x] Allow negative scale in Particles and CPUParticles #53852
• There are also proposals to introduce special abs_sign function to deal with this (Allow a way to select the behaviour of 0 in the sign() function.  godot-proposals#6854) but I've just fixed in place for now
• There might be some slightly more efficient way of doing this to avoid the branch

[akien-mga]

Thanks!