null analysis (#907)

Author: elharo

enforcer-rules/src/main/java/org/apache/maven/enforcer/rules/BannedRepositories.java

@@ -90,7 +90,7 @@ public void execute() throws EnforcerRuleException {
 
         String errMsg = repoErrMsg + pluginRepoErrMsg;
 
-        if (errMsg != null && !errMsg.isEmpty()) {
+        if (!errMsg.isEmpty()) {
             throw new EnforcerRuleException(errMsg);
         }
     }

enforcer-rules/src/main/java/org/apache/maven/enforcer/rules/RequirePluginVersions.java

@@ -401,7 +401,7 @@ private Plugin parsePluginString(String pluginString, String field) throws Enfor
                 throw new EnforcerRuleError("Invalid " + field + " string: " + pluginString);
             }
         } else {
-            throw new EnforcerRuleError("Invalid " + field + " string: " + pluginString);
+            throw new EnforcerRuleError("Invalid " + field + " null plugin string.");
         }
     }