fix(certinel): do not return nil client cert

[gofish]

The documentation and requirements for the methods GetCertificate and
GetClientCertificate are slightly different. The latter must not return
nil and should instead return an empty tls.Certificate, or else
trigger a nil-dereference in crypto/tls during the client handshake.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x65df22]

goroutine 39 [running]:
crypto/tls.(*clientHandshakeStateTLS13).sendClientCertificate(0xc0001ebe20, 0x0, 0x0)
    /usr/local/go/src/crypto/tls/handshake_client_tls13.go:537 +0x122
crypto/tls.(*clientHandshakeStateTLS13).handshake(0xc0001ebe20, 0xc000200100, 0x0)
    /usr/local/go/src/crypto/tls/handshake_client_tls13.go:91 +0x230
crypto/tls.(*Conn).clientHandshake(0xc00020a000, 0x0, 0x0)
    /usr/local/go/src/crypto/tls/handshake_client.go:196 +0x4d9
crypto/tls.(*Conn).Handshake(0xc00020a000, 0x0, 0x0)
    /usr/local/go/src/crypto/tls/conn.go:1340 +0xcc
net/http.(*persistConn).addTLS.func2(0x0, 0xc00020a000, 0x0, 0xc000186480)
    /usr/local/go/src/net/http/transport.go:1453 +0x42
created by net/http.(*persistConn).addTLS
    /usr/local/go/src/net/http/transport.go:1449 +0x1aa
FAIL    github.com/cloudflare/certinel  0.018s
ok      github.com/cloudflare/certinel/fswatcher        (cached)
FAIL