FieldNormalizerValue::rasterizeValue() respects cardinality === 1, but doesn't verify that this is true

While working on #2824851: EntityResource::patch() makes an incorrect assumption about entity keys, hence results in incorrect behavior, I ran into an explicable bug. A second item on a single-cardinality field was being set. Which should have caused tests to fail, because the field was different.

But, when normalized … the field was actually still the same! WTF, right?!

Turns out \Drupal\jsonapi\Normalizer\Value\FieldNormalizerValue::rasterizeValue() helpfully respects the cardinality of the field … but then also silently ignores any additional fields. An assertion verifying this precondition would have saved me tons of time.