Only run a fake master if MAIN_BRANCH and SOURCE_BRANCH are set

Author: Simon Hirst

Makefile

@@ -117,11 +117,9 @@ else ifeq "$(ARCH)" "s390x"
 endif
 
 # If this is a fake master build, push images to alternative location (pipeline wont consider these images GA candidates)
-ifeq "$(TRAVIS)" "true"
-ifneq "$(MAIN_BRANCH)" "$(SOURCE_BRANCH)"
+ifeq ($(shell [ "$(TRAVIS)" = "true" ] && [ -n $(MAIN_BRANCH) ] && [ -n $(SOURCE_BRANCH) ] && [ "$(MAIN_BRANCH)" != "$(SOURCE_BRANCH)" ] && echo "true"), true)
 	MQ_DELIVERY_REGISTRY_NAMESPACE="master-fake"
 endif
-endif
 
 # LTS_TAG is the tag modifier for an LTS container build
 LTS_TAG=