MavenBuilders: also disallow setting system properties with empty keys

maven-plugin/src/main/java/hudson/maven/Maven3Builder.java

@@ -113,8 +113,10 @@ public class Maven3Builder extends AbstractMavenBuilder implements DelegatingCal
             
             markAsSuccess = false;
 
-            // working around NPE when someone puts a null value into systemProps.
+            // working around NPE when someone puts a null value or empty keys into systemProps.
             for (Map.Entry<String,String> e : systemProps.entrySet()) {
+                if ("".equals(e.getKey()))
+                    throw new IllegalArgumentException("System property has an empty key");
                 if (e.getValue()==null)
                     throw new IllegalArgumentException("System property "+e.getKey()+" has a null value");
                 System.getProperties().put(e.getKey(), e.getValue());

maven-plugin/src/main/java/hudson/maven/MavenBuilder.java

@@ -154,8 +154,10 @@ public abstract class MavenBuilder extends AbstractMavenBuilder implements Deleg
             
             markAsSuccess = false;
 
-            // working around NPE when someone puts a null value into systemProps.
+            // working around NPE when someone puts a null value or empty keys into systemProps.
             for (Map.Entry<String,String> e : systemProps.entrySet()) {
+                if ("".equals(e.getKey()))
+                    throw new IllegalArgumentException("System property has an empty key");
                 if (e.getValue()==null)
                     throw new IllegalArgumentException("System property "+e.getKey()+" has a null value");
                 System.getProperties().put(e.getKey(), e.getValue());