Fix: Fix npe for propagation url settings

When upgrading from older versions of the plugin the propagationUrl tag
might be missing in
io.jenkins.plugins.bitbucketpushandpullrequest.config.BitBucketPPRPluginConfig.xml.
Since propagationUrl's value is checked for emptiness only this might
result in a NPE. This commit adds a check for null+emptiness.

src/main/java/io/jenkins/plugins/bitbucketpushandpullrequest/action/BitBucketPPRPullRequestServerAction.java

@@ -88,7 +88,7 @@ public BitBucketPPRPullRequestServerAction(@Nonnull BitBucketPPRPayload payload)
       }
     }
 
-    if (!globalConfig.getPropagationUrl().isEmpty()) {
+    if (globalConfig.isPropagationUrlSet()) {
       try {
         this.baseUrl = new URL(globalConfig.getPropagationUrl());
       } catch (MalformedURLException e) {

src/main/java/io/jenkins/plugins/bitbucketpushandpullrequest/config/BitBucketPPRPluginConfig.java

@@ -101,6 +101,10 @@ public String getPropagationUrl() {
     return propagationUrl;
   }
 
+  public boolean isPropagationUrlSet() {
+    return !isEmpty(propagationUrl);
+  }
+
   @DataBoundSetter
   public void setNotifyBitBucket(@CheckForNull boolean notifyBitBucket) {
     this.notifyBitBucket = notifyBitBucket;