project - zip and src check if empty fix

[0xc0170]

Regression from #2179. Fixes #2184.

Before:

python tools\project.py -m KL25Z -p 0 -i gcc_arm

 * KL25Z::gcc_arm      C:\Code\git_repo\github\mbed\.build\export\.temp\MBED_A1

With this patch:

python tools\project.py -m KL25Z -p 0 -i gcc_arm

  * KL25Z::gcc_arm     mbed\.build\export\MBED_A1_gcc_arm_KL25Z.zip

@sarahmarshy @theotherjimmy @screamerbg

[0xc0170]

cc @ohagendorf

[sarahmarshy]

I also pushed a fix, but this looks good. Sorry about that :/

[0xc0170]

@sarahmarshy Please test , I checked the previous logic. if this not bool is correct, and works also for --source

[ohagendorf]

LGTM

[sarahmarshy]

Tested with and without --source. Looks good.