fix negotiation fail when empty sub-auth types received

[avost]

When a TightVNC server configured to allow no-auth connection, it may sends (the recent TightVNC server actually sends) an empty 'sub-auth' capabilities list at tight security type sub-auth negotiation phase. In this case noVNC viewer fails on negotiation with ''Unsupported tiny auth scheme:" message.
Fix this to assume 'no-auth' authentication subtype selected when received an empty auth capabilities list.

[DirectXMan12]

For future reference, the RFB proto says this about TIGHT auth:

If number-of-auth-types is non-zero, the client has to request an authentication type from the list with an authentication scheme request
...
If number-of-auth-types is zero, the protocol the proceeds directly at security type None.

(https://github.com/rfbproto/rfbproto/blob/master/rfbproto.rst#723tight-security-type), which matches the change in this PR.

[DirectXMan12]

Just needs a rebase, which I can do