devtool: default to empty string, if LIC_FILES_CHKSUM is not available

devtool: default to empty string, if LIC_FILES_CHKSUM is not available

[Bug 13971] https://bugzilla.yoctoproject.org/show_bug.cgi?id=13971

A recipe using LICENSE=CLOSED would normally not have LIC_FILES_CHKSUM
set.
If LIC_FILES_CHKSUM was not set, it would cause `devtool upgrade` to fail.

This fixes the issue by defaulting to an empty string if
LIC_FILES_CHKSUM is not found.

[YOCTO #13971]

Signed-off-by: Bjarne Michelsen <bjarne_michelsen@hotmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>