Fix slash commands null guard

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

src/SlashCommands.tsx

@@ -1049,7 +1049,7 @@ export function parseCommandString(input) {
     // trim any trailing whitespace, as it can confuse the parser for
     // IRC-style commands
     input = input.replace(/\s+$/, '');
-    if (input[0] !== '/') return null; // not a command
+    if (input[0] !== '/') return {}; // not a command
 
     const bits = input.match(/^(\S+?)(?: +((.|\n)*))?$/);
     let cmd;