Ensure runner pane is present before sending lines

Closes #8 - Only send line(s) when runner pane is open

plugin/vim-tmux-runner.vim

@@ -374,6 +374,7 @@ function! s:PrepareLines(lines)
 endfunction
 
 function! s:SendTextToRunner(lines)
+    call s:EnsureRunnerPane()
     let prepared = s:PrepareLines(a:lines)
     let joined_lines = join(prepared, "\r") . "\r"
     let send_keys_cmd = s:TargetedTmuxCommand("send-keys", s:runner_pane)