commands: ignore empty input

Prior to this, when sending nothing, senpai throws the following
nonsense at you:

    "": usage:

Now empty input is ignored, preventing this from appearing
(whitespace-only message should still work).

1 files changed, 4 insertions, 0 deletions

diff --git a/commands.go b/commands.go
index fee0571..621b034 100644
--- a/commands.go
+++ b/commands.go
@@ -350,6 +350,10 @@ func parseCommand(s string) (command, args string) {
 func (app *App) handleInput(buffer, content string) error {
 	cmdName, rawArgs := parseCommand(content)
 
+	if content == "" {
+		return nil
+	}
+
 	var chosenCMDName string
 	var ok bool
 	for key := range commands {