[TD-4158]<fix>: taosdemo cmake failed with low version git. (#6107)

src/kit/taosdemo/CMakeLists.txt

@@ -10,7 +10,11 @@ IF (GIT_FOUND)
         COMMAND ${GIT_EXECUTABLE} log --pretty=oneline -n 1 ${CMAKE_CURRENT_LIST_DIR}/taosdemo.c
         RESULT_VARIABLE RESULT
         OUTPUT_VARIABLE TAOSDEMO_COMMIT_SHA1)
-    STRING(SUBSTRING "${TAOSDEMO_COMMIT_SHA1}" 0 7 TAOSDEMO_COMMIT_SHA1)
+    IF ("${TAOSDEMO_COMMIT_SHA1}" STREQUAL "")
+        MESSAGE("taosdemo's latest commit in short is:" ${TAOSDEMO_COMMIT_SHA1})
+    ELSE ()
+        STRING(SUBSTRING "${TAOSDEMO_COMMIT_SHA1}" 0 7 TAOSDEMO_COMMIT_SHA1)
+    ENDIF ()
     EXECUTE_PROCESS(
         COMMAND ${GIT_EXECUTABLE} status -z -s ${CMAKE_CURRENT_LIST_DIR}/taosdemo.c
         RESULT_VARIABLE RESULT