Test server can overwrite last heartbeat details when not on fail request

[cretz]

Expected Behavior

Test server should not overwrite last heartbeat details on activity failure unless it is actually set

Actual Behavior

As of #2354, the last heartbeat details are always overridden regardless of whether set which, while that's ok for Java, is not for Core-based SDKs (at least until temporalio/sdk-core#293 is done)