Set Runtime.currentStepTime to a number by default instead of null

src/engine/runtime.js

@@ -321,7 +321,7 @@ class Runtime extends EventEmitter {
          * WORK_TIME.
          * @type {!number}
          */
-        this.currentStepTime = null;
+        this.currentStepTime = 1000 / 30;
 
         // Set an intial value for this.currentMSecs
         this.updateCurrentMSecs();

test/unit/engine_runtime_tw.js

@@ -213,3 +213,10 @@ test('getNumberOfCloudVariables', t => {
 
     t.end();
 });
+
+test('currentStepTime default value', t => {
+    const rt = new Runtime();
+    t.type(rt.currentStepTime, 'number');
+    t.ok(rt.currentStepTime > 0);
+    t.end();
+});