devtool: fix devtool finish when gitmodules file is empty

When a .gitmodules file exists but is empty then devtool finish fails.
Add an additional check for this.

[YOCTO #14999]

Signed-off-by: Thomas Roos <throos@amazon.de>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>