fix: avoid treating empty values as valid CSV input

[miraclx]

While using Axum, I noticed If-Match & If-None-Match would always default to Some("") making it difficult to know if the user passed the headers as If-Match: or not at all.

I looked into the code, and it turns out the logic for consuming the values iterator defaults to an empty byte array (ergo, empty string)

I chose this peeking method and short-circuiting before calling collect to avoid more complex patterns.

And tags in If-Match & If-None-Match (depending on EntityTagRange) weren't being validated.

This fixes that.

[paolobarbolini]

This will also fix my issue #148

[evgenykobz]

Second this: this bug breaks OptionalFromRequestParts implementations.