add warning about empty message
add warning about empty message

rexquigg/brainwallet,A-Zak/brainwallet.github.io,brainwalletX/brainwalletX.github.io,jamescarter-le/brainwallet.github.io,jhoenicke/brainwallet.github.io,jamescarter-le/brainwallet.github.io,btcspry/brainwallet.github.io,Taiiwo/bitcoin-signature-tool,sipak/brainwallet,brainwalletX/brainwalletX.github.io,Colored-Coins/brainwallet.github.io,jhoenicke/brainwallet.github.io,brainwallet/brainwallet.github.io,2XL/brainwallet.github.io,Taiiwo/bitcoin-signature-tool,nvoron23/brainwallet.github.io,sipak/brainwallet,btcspry/brainwallet.github.io,nvoron23/brainwallet.github.io,2XL/brainwallet.github.io,Colored-Coins/brainwallet.github.io,A-Zak/brainwallet.github.io