fix(Makefile): use string value checks instead of ifndef (#9226)

a variable can be defined, and still have an empty string value.

Author: muncus

Makefile

@@ -15,7 +15,11 @@ dir ?= $(shell pwd)
 
 # GOOGLE_SAMPLES_PROJECT takes precedence over GOOGLE_CLOUD_PROJECT
 PROJECT_ID = ${GOOGLE_SAMPLES_PROJECT}
-PROJECT_ID ?= ${GOOGLE_CLOUD_PROJECT}
+
+ifeq ("${PROJECT_ID}", "")
+PROJECT_ID = ${GOOGLE_CLOUD_PROJECT}
+endif
+
 # export our project ID as GOOGLE_CLOUD_PROJECT in the action environment
 override GOOGLE_CLOUD_PROJECT := ${PROJECT_ID}
 export GOOGLE_CLOUD_PROJECT
@@ -38,7 +42,7 @@ lint:
 	mvn -P lint checkstyle:check
 
 check-env:
-ifndef PROJECT_ID
+ifeq ("${PROJECT_ID}", "")
 	$(error At least one of the following env vars must be set: GOOGLE_SAMPLES_PROJECT, GOOGLE_CLOUD_PROJECT.)
 endif