ci: skip Bintray publish also if BINTRAY_USER is null

Formerly it was only skipped if it was explicitly set to
SKIP_BINTRAY_PUBLISH. This is retained for backward compatibility.

Author: BlasiusSecundus

.github/workflows/ci.yml

@@ -63,7 +63,7 @@ jobs:
       - name: Make gradlew executable
         run: chmod +x ./gradlew
       - name: Gradle Publish Snapshot
-        if: env.BINTRAY_USER != 'SKIP_BINTRAY_PUBLISH'
+        if: env.BINTRAY_USER != 'SKIP_BINTRAY_PUBLISH' && env.BINTRAY_USER != null
         env:
           BINTRAY_USER: ${{ secrets.BINTRAY_USER }}
           BINTRAY_PASS: ${{ secrets.BINTRAY_PASSWORD }}