fixes gradle env-var interpretation issue on macos (#298)

Author: bobrossthepainter

build.gradle

@@ -55,7 +55,10 @@ test {
     systemProperty "kubernetes.disable.autoConfig", "true"
 
     environment 'TESTCONTAINERS_REUSE_ENABLE', 'true'
-    environment 'CONTAINER_FILTER', System.getenv("CONTAINER_FILTER")
+    
+    if(System.getenv("CONTAINER_FILTER")) {
+        environment 'CONTAINER_FILTER', System.getenv("CONTAINER_FILTER")
+    }
 
     useJUnitPlatform()