Sensor.timestamp: Account for "get value from latest reading" returning null (#480)

This addresses a regression introduced by #469. The current algorithm
assumes that "get value from latest reading" never returns null, which is
not the case.

Return null when it does instead of passing an invalid value to "relative
high resolution time".

Author: Raphael Kubo da Costa

index.bs

@@ -1122,9 +1122,10 @@ with the internal slots described in the following table:
     The {{Sensor/timestamp}} getter steps are:
 
     1. Let |global| be [=this=]'s [=relevant global object=].
-    2. Let |unsafeTimestamp| be the result of invoking [=get value from latest reading=] with
+    1. Let |unsafeTimestamp| be the result of invoking [=get value from latest reading=] with
        [=this=] and "timestamp" as arguments.
-    3. Return [=relative high resolution time=] with |unsafeTimestamp| and |global|.
+    1. If |unsafeTimestamp| is null, return null.
+    1. Otherwise, return [=relative high resolution time=] with |unsafeTimestamp| and |global|.
 </div>
 
 ### Sensor.start() ### {#sensor-start}