CDRIVER-4498 return error if `masterKey` is set, but `provider` is not set (#1259)

* add CSE Prose Test 16: Case 2

* return error if `provider` is not set when `master_key` is set

* Simplify assertion of provider

Co-authored-by: Ezra Chung <88335979+eramongodb@users.noreply.github.com>

* update commented test name to match specification

* use empty doc for masterKey to match specification

* use ASSERT_WITH_MSG

* clarify in docs that current master key is used when provider is NULL

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Co-authored-by: Ezra Chung <88335979+eramongodb@users.noreply.github.com>