[Execution State] Payload.Equals() can crash if p is nil, and it should return true when comparing empty payload and nil payload

[fxamacker]

Problem

There appears to be two bugs:

• func (p *Payload) Equals(other *Payload) bool can crash if p is nil. Although this scenario seems to be unlikely to happen the way the function is currently used, it can become a problem if we use this function differently.

• nil payload doesn't equal empty payload, but they should be equal.

Solution

These are being fixed as part of upcoming PR resolving issues #1747 and #1748.