Fall back to a cmake generated compile hash when git failed

Even if we have git, the source tree might not be a git repository, or
git might fail for various reasons. So always fall back to the cmake-
generated QML compile hash when it's empty after trying the .tag file
and git.

Amends a1bba5cd3d.

Pick-to: 6.2 6.3
Change-Id: Ie9f5642bbfc0cead40138d1f92fe3d673f6dc372
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

cmake/QtDeclarativeSetup.cmake

@@ -16,7 +16,9 @@ function(qt_declarative_write_tag_header target_name)
             OUTPUT_VARIABLE QML_COMPILE_HASH
             OUTPUT_STRIP_TRAILING_WHITESPACE
             WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}")
-    else()
+    endif()
+    string(LENGTH "${QML_COMPILE_HASH}" QML_COMPILE_HASH_LENGTH)
+    if(QML_COMPILE_HASH_LENGTH EQUAL 0)
         set(sources_hash "")
         file(GLOB_RECURSE qtqml_source_files "${CMAKE_CURRENT_SOURCE_DIR}/*")
         foreach(file IN LISTS qtqml_source_files)