Autogen: Fix empty uic executable string. CMake 3.13.0 backport.
In CMake 3.13.0 when a target has AUTOUIC
enabled but doesn't
link against Qt5Widgets, the uic executable string is empty and
an error string "The uic executable "" does not exist" is generated.
In CMake 3.12 AUTOUIC
was silently disabled instead.
This patch fixes the regression and restores the behavior of CMake 3.12.
Fixes: #18630 (closed)