fixed NPE on empty props

Author: fehguy

modules/swagger-codegen-maven-plugin/src/main/java/io/swagger/codegen/plugin/CodeGenMojo.java

@@ -131,7 +131,12 @@ public void execute() throws MojoExecutionException {
 
         if (environmentVariables != null) {
             for(String key : environmentVariables.keySet()) {
-                System.setProperty(key, environmentVariables.get(key));
+                String value = environmentVariables.get(key);
+                if(value == null) {
+                    // don't put null values
+                    value = "";
+                }
+                System.setProperty(key, value);
             }
         }