cmake: fix build failure when git repo is initialized but no first commit

Closes https://github.com/espressif/esp-idf/issues/3920

tools/cmake/third_party/GetGitRevisionDescription.cmake.in

@@ -24,10 +24,12 @@ if(HEAD_CONTENTS MATCHES "ref")
 	if(EXISTS "@GIT_DIR@/${HEAD_REF}")
 		configure_file("@GIT_DIR@/${HEAD_REF}" "@GIT_DATA@/head-ref" COPYONLY)
 	else()
-		configure_file("@GIT_DIR@/packed-refs" "@GIT_DATA@/packed-refs" COPYONLY)
-		file(READ "@GIT_DATA@/packed-refs" PACKED_REFS)
-		if(${PACKED_REFS} MATCHES "([0-9a-z]*) ${HEAD_REF}")
-			set(HEAD_HASH "${CMAKE_MATCH_1}")
+		if(EXISTS "@GIT_DIR@/packed-refs")
+			configure_file("@GIT_DIR@/packed-refs" "@GIT_DATA@/packed-refs" COPYONLY)
+			file(READ "@GIT_DATA@/packed-refs" PACKED_REFS)
+			if(${PACKED_REFS} MATCHES "([0-9a-z]*) ${HEAD_REF}")
+				set(HEAD_HASH "${CMAKE_MATCH_1}")
+			endif()
 		endif()
 	endif()
 else()