fix: fixed gts issues

Author: djonathascardoso

packages/opencensus-core/src/trace/model/span-base-model.ts

@@ -72,23 +72,38 @@ export abstract class SpanBaseModel implements Span {
      * "Zulu" format.
      */
     get startTime(): Date {
-      if (this.clock) return this.clock.startTime;
+      if (!this.clock) {
+        debug("calling startTime() on null clock");
+        return null;
+      }
+      
+      return this.clock.startTime;
     }
 
     /**
      * Gives a timestap that indicates the span's end time in RFC3339 UTC
      * "Zulu" format.
      */
     get endTime(): Date {
-      if (this.clock) return this.clock.endTime;
+      if (!this.clock) {
+        debug("calling endTime() on null clock");
+        return null;
+      }
+
+      return this.clock.endTime;
     }
 
     /**
      * Gives a timestap that indicates the span's duration in RFC3339 UTC
      * "Zulu" format.
      */
     get duration(): number {
-      if (this.clock) return this.clock.duration;
+      if (!this.clock) {
+        debug("calling duration() on null clock");
+        return null;
+      }
+
+      return this.clock.duration;
     }
 
     /** Gives the TraceContext of the span. */