cmake: Handle empty SOF_SRC_HASH_LONG

SOF_SRC_HASH always must have integer value, because of usage
them to initialize global variable in source code.
Variables, which may be empty should be used inside quotation
to prevent cmake incorect number of arguments error.

Signed-off-by: Karol Trzcinski <karolx.trzcinski@linux.intel.com>

scripts/cmake/version.cmake

@@ -84,7 +84,11 @@ if(EXISTS ${SOF_ROOT_SOURCE_DIRECTORY}/.git/)
 	string(SUBSTRING ${SOF_SRC_HASH_LONG} 0 8 SOF_SRC_HASH)
 	message(STATUS "Source content hash: ${SOF_SRC_HASH}")
 else()
-	string(SUBSTRING ${GIT_LOG_HASH} 0 8 SOF_SRC_HASH)
+	if("${GIT_LOG_HASH}")
+		string(SUBSTRING "${GIT_LOG_HASH}" 0 8 SOF_SRC_HASH)
+	else()
+		set(SOF_SRC_HASH "0")
+	endif()
 	message(WARNING "Source content hash not computed without git, using GIT_LOG_HASH instead")
 endif()