[GRASS] cmake fix

Author: blazek

cmake/FindGRASS.cmake

@@ -78,7 +78,9 @@ MACRO (CHECK_GRASS G_PREFIX)
             ERROR_VARIABLE GRASS_TMP_ERROR
             OUTPUT_VARIABLE GRASS_OFF_T_SIZE${GRASS_FIND_VERSION}
           )
-          STRING(STRIP ${GRASS_OFF_T_SIZE${GRASS_FIND_VERSION}} GRASS_OFF_T_SIZE${GRASS_FIND_VERSION})
+          IF ( NOT ${GRASS_OFF_T_SIZE${GRASS_FIND_VERSION}} STREQUAL "" )
+            STRING(STRIP ${GRASS_OFF_T_SIZE${GRASS_FIND_VERSION}} GRASS_OFF_T_SIZE${GRASS_FIND_VERSION})
+          ENDIF()
           #MESSAGE(STATUS "GRASS_OFF_T_SIZE${GRASS_FIND_VERSION} = ${GRASS_OFF_T_SIZE${GRASS_FIND_VERSION}}")
         ENDIF( (GRASS_MAJOR_VERSION${GRASS_FIND_VERSION} EQUAL 7) AND (GRASS_MINOR_VERSION${GRASS_FIND_VERSION} GREATER 0) )