x-remove-whitespace trips itself up on empty attributes

[parnikkapore]

Currently:

((compose xexpr->string x-remove-whitespace string->xexpr) "<dog type=\" \"> <siblings /> </dog>")

results in the following abomination:

x-remove-whitespace: broke its own contract
  promised: xexpr?
  produced: '(dog ((type)) (siblings ()))

The space in the attribute's value is removed, which results in an invalid xexpr.

[parnikkapore]

Fixing this will likely require a smarter xexpr lexer inside libquiche...

[parnikkapore]

xml/private/xexpr.rkt:118 looks interesting