build: don't clean obj.target directory if it doesn't exist

This can happen to ninja builds, which would then throw an
error instead of being a noop if the command is executed.

Makefile

@@ -266,7 +266,9 @@ coverage-build-js:
 
 .PHONY: coverage-test
 coverage-test: coverage-build
-	$(FIND) out/$(BUILDTYPE)/obj.target -name "*.gcda" -type f -exec $(RM) {} \;
+	@if [ -d "out/Release/obj.target" ]; then \
+		$(FIND) out/$(BUILDTYPE)/obj.target -name "*.gcda" -type f -exec $(RM) {}; \
+	fi
 	-NODE_V8_COVERAGE=coverage/tmp \
 		TEST_CI_ARGS="$(TEST_CI_ARGS) --type=coverage" $(MAKE) $(COVTESTS)
 	$(MAKE) coverage-report-js