FIX NPE in tomcat plugin ForwardInterceptor (#1562)

apm-sniffer/apm-sdk-plugin/tomcat-7.x-8.x-plugin/src/main/java/org/apache/skywalking/apm/plugin/tomcat78x/ForwardInterceptor.java

@@ -37,7 +37,7 @@ public class ForwardInterceptor implements InstanceMethodsAroundInterceptor, Ins
         if (ContextManager.isActive()) {
             AbstractSpan abstractTracingSpan = ContextManager.activeSpan();
             Map<String, String> eventMap = new HashMap<String, String>();
-            eventMap.put("forward-url", (String)objInst.getSkyWalkingDynamicField());
+            eventMap.put("forward-url", objInst.getSkyWalkingDynamicField() == null ? "" : String.valueOf(objInst.getSkyWalkingDynamicField()));
             abstractTracingSpan.log(System.currentTimeMillis(), eventMap);
             ContextManager.getRuntimeContext().put(Constants.FORWARD_REQUEST_FLAG, true);
         }