Improve validation on msg_server methods

[giorgionocera]

In the nft module, inside the methods of the msg_server, we should improve the validations.

In particular, we should include validation on:

• CreateCollection method: on data of the collection (Symbol, Name, Uri can also be empty strings, and, UpdateAuthority address can also be an invalid string). Even the ValidateBasic function does not perform these checks. Deriving from issue CreateCollection msg_server #193
• UpdateMetadata method: on data of the collection (Name, Uri can also be empty strings, SellerFeeBasisPoints can be any positive number, and, Creators addresses can also be invalid strings). Even the ValidateBasic function does not perform these checks (only checks that SellerFeeBasisPoints is not greater than 100). Deriving from issue UpdateMetadata msg_server #194