Figure out how to handle calls to isEmpty on Array[Char] for JDK 15 and Scala 2.13.4 compat

[smarter]

See scala/scala#9239, if Scala 2 decides to add a macro method to Array.scala, this will mean that we won't be able to call isEmpty on Array (on any JDK version) until we add more special-cases in dotty itself to handle it, so this is a blocker for M1.

[smarter]

Looks like we'll thankfully be able to have with a purely library-based solution: scala/scala#9239 (comment), which means we don't have to do anything in dotty.