yocto-compat-layer.py: Fix trace when layers can't be processed

yocto-compat-layer.py: Fix trace when layers can't be processed

When all of the requested layers have unsatisfied dependencies, an error
can occur.  Check for the condition to avoid the traceback:

Traceback (most recent call last):
  File "../scripts/yocto-compat-layer.py", line 203, in <module>
    ret =  main()
  File "../scripts/yocto-compat-layer.py", line 194, in main
    if not results[layer_name].wasSuccessful():
AttributeError: 'NoneType' object has no attribute 'wasSuccessful'

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>