Fix NPE in Signature.toString()

Author: karllessard

tensorflow-core/tensorflow-core-api/src/main/java/org/tensorflow/Signature.java

@@ -208,9 +208,10 @@ public Set<String> outputNames() {
 
   @Override
   public String toString() {
-    StringBuilder strBuilder = new StringBuilder("Signature for \"" + key + "\":\n");
-    if (!methodName().isEmpty()) {
-      strBuilder.append("\tMethod: \"").append(methodName()).append("\"\n");
+    StringBuilder strBuilder = new StringBuilder("Signature for \"" + key +"\":\n");
+    String methodName = methodName();
+    if (methodName != null && !methodName.isEmpty()) {
+      strBuilder.append("\tMethod: \"").append(methodName).append("\"\n");
     }
     if (signatureDef.getInputsCount() > 0) {
       strBuilder.append("\tInputs:\n");