CMake: Fix running qt-configure-module without arguments

We need to make sure to create a config.opt file, even if it's empty.
This is the same fix we did for the .bat file in
dad9550305831784f124c44d08e1e20983415bda.

Change-Id: I79c507cfe0f9517d3033a773d3a5cf69eb13cfe6
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

bin/qt-configure-module.in

@@ -21,7 +21,7 @@ if [ ! -f "$module_root/CMakeLists.txt" ]; then
 fi
 
 optfile=config.opt
-if [ -f "$optfile" ]; then rm "$optfile"; fi
+echo > "$optfile"
 for arg in "$@"; do
 echo $arg >> "$optfile"
 done