oidcclaimquery: better error handling for casting

[MustafaSaber]

Describe the bug

if oidcclaimsquery get a jwt token that doesn't have sub for any reason it will fail to cast nil to string.

skipper/filters/auth/oidc_introspection.go

Line 134 in 00ff742

sub := token.Claims["sub"].(string)

To Reproduce

use a token without sub

Expected behavior

Better error handling instead of failing without clue what is happening.