fix: check emptyness of GO_TEST_IMAGE

with component based new template, `GO_TEST_IMAGE` can't be null but can
be empty.

So instead of checkig if it's, let's check it's empty

closes #34

Signed-off-by: Sylvain Desbureaux <sylvain.desbureaux@orange.com>

templates/gitlab-ci-golang.yml

@@ -512,7 +512,7 @@ go-test:
       - "$GO_PROJECT_DIR/reports/go-coverage.*"
   rules:
     # if $GO_TEST_IMAGE set
-    - if: '$GO_TEST_IMAGE == null'
+    - if: '$GO_TEST_IMAGE == ""'
       when: never
     - !reference [.test-policy, rules]
 
@@ -540,7 +540,7 @@ go-build-test:
       - "$GO_PROJECT_DIR/reports/go-coverage.*"
   rules:
     # if $GO_TEST_IMAGE not set
-    - if: '$GO_TEST_IMAGE == null'
+    - if: '$GO_TEST_IMAGE == ""'
 
 go-ci-lint:
   extends: .go-base