providers: don't send empty body/usage lines to IRC

[dgw]

There's no effect visible to the user, but we can eliminate an unnecessary error (412) from the IRC server.

Before:

2023-11-09 21:49:41,145 >>      "PRIVMSG dgw :Manage Sopel's blocking features.\r\n"
2023-11-09 21:49:41,146 >>      'PRIVMSG dgw :\r\n'
2023-11-09 21:49:41,147 >>      'PRIVMSG dgw :Full argspec: `list [nick|host]` or `[add|del] [nick|host] pattern`\r\n'
2023-11-09 21:49:41,148 >>      'PRIVMSG dgw :e.g. .blocks add nick sp(a|4)mb(o|0)t\\d*, .blocks add host some\\.malicious\\.network or .blocks del nick falsep0sitive\r\n'
2023-11-09 21:49:41,193 <<      ':irc.rizon.io 412 SopelTest :No text to send\r\n'

After:

2023-11-09 22:07:00,389 >>      "PRIVMSG dgw :Manage Sopel's blocking features.\r\n"
2023-11-09 22:07:00,390 >>      'PRIVMSG dgw :Full argspec: `list [nick|host]` or `[add|del] [nick|host] pattern`\r\n'
2023-11-09 22:07:00,391 >>      'PRIVMSG dgw :e.g. .blocks add nick sp(a|4)mb(o|0)t\\d*, .blocks add host some\\.malicious\\.network or .blocks del nick falsep0sitive\r\n'

[Exirel]

OK I'm going to merge that without the required tests... which I'm going to write myself.