[bugfix] permit empty matches

[tstat]

Overview

Fix a bug in the pattern match coverage checker's desugarer that interpreted an empty match as a wildcard match.

Simple test

Here is a simple transcript test when this diff is applied to #5249:

inhabited types are rejected

unique type T = A | B

bonk : T -> ()
bonk x = match x with

  Loading changes detected in scratch.u.

  Pattern match doesn't cover all possible cases:
        4 | bonk x = match x with
    
  
  Patterns not matched:
  
    * A
    * B

uninhabited types are accepted

unique type V =

bonk : V -> ()
bonk x = match x with

  Loading changes detected in scratch.u.

  I found and typechecked these definitions in scratch.u. If you
  do an `add` or `update`, here's how your codebase would
  change:
  
    ⍟ These new definitions are ok to `add`:
    
      type V
      bonk : V -> ()

Test coverage

Just running the above transcript

[tstat]

This is the bug, an empty set of match cases is not a Leaf, it is an empty tree.

[aryairani]

🙏 🏆 Thank you!

[aryairani]

Here is a simple transcript test when this diff is applied to #5249:

cc @ChrisPenner

[ChrisPenner]

Thanks @tstat ! Much appreciated