Fix panic with var in glob

[gavin-ts]

Summary

Fixes a panic that could happen using a var in a glob.

Details

• fixes panic using var in glob #1642
• adds var_in_glob test
• updates Equals check to consider nil MapKeys (spread substitution into map has no map key)
• would occur when comparing RefContexts to find the glob context

substitution can have nil MapKey

(d2ast.MapNodeBox) {
 Comment: (*d2ast.Comment)(<nil>),
 BlockComment: (*d2ast.BlockComment)(<nil>),
 Substitution: (*d2ast.Substitution)(0x14000519140)({
  Range: (d2ast.Range) 10:3,
  Spread: (bool) true,
  Path: ([]*d2ast.StringBox) (len=1 cap=1) {
   (*d2ast.StringBox)(0x140003f4c00)({
    UnquotedString: (*d2ast.UnquotedString)(0x140004285b0)({
     Range: (d2ast.Range) 10:8,
     Value: ([]d2ast.InterpolationBox) (len=1 cap=1) {
      (d2ast.InterpolationBox) {
       String: (*string)(0x14000f485c0)((len=1) "v"),
       StringRaw: (*string)(0x14000f485d0)((len=1) "v"),
       Substitution: (*d2ast.Substitution)(<nil>)
      }
     },
     Pattern: ([]string) <nil>
    }),
    DoubleQuotedString: (*d2ast.DoubleQuotedString)(<nil>),
    SingleQuotedString: (*d2ast.SingleQuotedString)(<nil>),
    BlockString: (*d2ast.BlockString)(<nil>)
   })
  }
 }),
 Import: (*d2ast.Import)(<nil>),
 MapKey: (*d2ast.Key)(<nil>)
}

[alixander]

can you also get D2OracleEquals?