AbstractTimingSource: setTimeAndBpm: make bpm optional

src/Playback/TimingSources/AbstractTimingSource.ts

@@ -12,9 +12,9 @@ export class AbstractTimingSource implements ITimingSource {
         }
         this.reset();
     }
-    public setTimeAndBpm(timestamp: Fraction, bpm: number): void {
+    public setTimeAndBpm(timestamp: Fraction, bpm?: number): void {
         this.anchorTimestamp = Fraction.createFromFraction(timestamp);
-        if (this.Settings !== undefined) {
+        if (this.Settings !== undefined && bpm) {
             this.Settings.BeatsPerMinute = bpm;
         }
         this.reset();