devtool: update-recipe: check output before treating it as a string
As of the move to Python 3 and the fixes we applied at that time,
bb.process.run() will return a byte array of length 0 rather than an
empty string if the output is empty. That may be a bug that we should
fix, but for now it's easiest to just check the result here before
treating it as a string. This fixes running "devtool update-recipe" or
"devtool finish" on a recipe which has no source tree, for example
initramfs-framework.
Fixes [YOCTO #10563].
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
