Firestore security rule emulator - get and getAfter throwing an exception on non existant doc

### [REQUIRED] Environment info

**firebase-tools:**7.16.1

**Platform:**Windows

### [REQUIRED] Test case

Just write a rule using a get or getAfter on a non existant doc.
For instance:
allow update: if get(/databases/$(database)/documents/doc/doesnot/exist) == null;

### [REQUIRED] Steps to reproduce

See above

### [REQUIRED] Expected behavior

As per the doc get and getAfter should return null. this is what the online simulator does.

### [REQUIRED] Actual behavior

An exception is thrown
Service call error. Function: [get], Argument: [path_value {
...}