Shell: Properly handle parser syntax errors

In the case of a syntax error the shell parser prints an error message
to stderr and returns an empty Vector<Command> - in that case we
shouldn't try to determine whether or not we can continue parsing but
abort immediately - is_complete() expects that *something* was parsed
successfully.

Fixes #2251.

Shell/main.cpp

@@ -857,6 +857,9 @@ static ExitCodeOrContinuationRequest run_command(const StringView& cmd)
 
     auto commands = Parser(cmd).parse();
 
+    if (!commands.size())
+        return 1;
+
     auto needs_more = is_complete(commands);
     if (needs_more != ExitCodeOrContinuationRequest::Nothing)
         return needs_more;