Wrong type in JWE_EC.dec_setup

[vernans]

Hello cryptojwt team,
Thanks for your work, it usually works like a charm.
I found an issue in cryptojwt.jwe.jwe_ec.JWE_EC.dec_setup.

JWTConnect-Python-CryptoJWT/src/cryptojwt/jwe/jwe_ec.py

Line 147 in 727bb9b

apu = apv = ""

apu and apv are assigned "" when they are declared. When there is no other value in the header for them subsequently ecdh_derive_key crashes because it's trying to concatenate these strings with bytes. I guess they should be assigned a b"".

As I know your time also doesn't grow on trees let me know if I can help you with providing a PR for that.

[jschlyter]

Please create a PR and I'll process it swiftly.