linux: GlobalMenu: UBUNTU_MENUPROXY with >1 char

When testing if $UBUNTU_MENUPROXY is set, we should also check if it
contains more than one character. Avoid cases when $UBUNTU_MENUPROXY is
set to 0 or 1.
Merge pull request #708 from matttbe/globalmenu

linux: GlobalMenu: UBUNTU_MENUPROXY with >1 char