Hob: check if warnings are related to package build

If warnings come from recipes parsing
and not from package build, 'parent' object
will be None; so don't update the color for it.

[YOCTO #5621]

Signed-off-by: Valentin Popa <valentin.popa@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>