Fix CMake bundling error on macOS

Author: dakcarto

cmake/MacBundleMacros.cmake

@@ -233,4 +233,6 @@ FILE (GLOB QGPLUGLIST "${QPLUGDIR}/*.so")
 FILE (GLOB QGPYLIST "${QGISPYDIR}/qgis/*.so")
 FILE (GLOB QGAPPLIST RELATIVE "${QBINDIR}" "${QBINDIR}/q*.app")
 FILE (GLOB QGRASSEXECLIST RELATIVE "${QLIBXDIR}/grass" "${QLIBXDIR}/grass/*/*")
-STRING(REPLACE ".app" ";" QGAPPLIST ${QGAPPLIST})
+IF (QGAPPLIST)
+  STRING(REPLACE ".app" ";" QGAPPLIST ${QGAPPLIST})
+ENDIF (QGAPPLIST)