Allow for empty message posts if the poll is specified

[RiccardoM]

Context

Currently the user cannot create an empty message post which contains a message, because the message is marked optional only if the medias are not empty.

We should definitely allow the creation of empty messages posts if they contain a poll.

Solution

In order to allow this, we should reduce the tx posts create command required arguments from 2 to 1, making the message optional.

Then we need to check in both the ValidateBasic method of MsgCreatePost and Validate of Post if the post has either a message, a poll or at least a media to be valid.