Fix intercept TypeApply in explicit nulls

[noti0na1]

Use erased-type semantic to intercept TypeApply in explicit nulls, so the following example doesn't have warning about "the type test for argType cannot be checked at runtime".

class Symbol {
  type ThisName
}

type TermSymbol = Symbol { type ThisName = String }

def f(x: TermSymbol | Null) =
  x match
    case key: TermSymbol => 1
    case null => 0