Join orders to customers without matching blanks

The rules

These rules decide what a correct result is. Your answer is checked on this dataset and on hidden datasets that follow the same rules.

Inputs
  orders(id, customer_key) — text. id is unique.
  customers(customer_key, segment) — text. Present keys are trimmed and unique. One row has a missing key.

Rules
  customer_key: remove ASCII whitespace (space, tab, CR, LF) at both ends; an empty result is missing.
  Add segment from customers by customer_key. Keep every order.
  An order whose key is missing, or has no matching customer, gets a missing segment.
  A missing key never matches a missing key.

Output (row order does not matter)
  clean(id text, customer_key text, segment text)