runtime(osc52): A few minor fixes

- If g:osc52_disable_paste is enabled, then return an empty list instead
  of a list with a single empty string.
- use `echo` instead of `echom` when printing osc waiting message

closes: #19002

Signed-off-by: Foxe Chen <chen.foxe@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/pack/dist/opt/osc52/autoload/osc52.vim

@@ -12,7 +12,7 @@ enddef
 var sent_message: bool = false
 
 def OSCMessage(id: number)
-  echom "Waiting for OSC52 response... Press CTRL-C to cancel"
+  echo "Waiting for OSC52 response... Press CTRL-C to cancel"
   sent_message = true
 enddef
 
@@ -20,7 +20,7 @@ export def Paste(reg: string): tuple<string, list<string>>
   # Check if user has indicated that the terminal does not support OSC 52 paste
   # (or has disabled it)
   if get(g:, 'osc52_disable_paste', 0)
-    return ("c", [""])
+    return ("c", [])
   endif
 
   # Some terminals like Kitty respect the selection type parameter on both X11

src/testdir/dumps/Test_osc52_paste_01.dump

@@ -17,4 +17,4 @@
 |~| @73
 |~| @73
 |~| @73
-|W+0#0000000&|a|i|t|i|n|g| |f|o|r| |O|S|C|5|2| |r|e|s|.@2|P|r|e|s@1| |C|T|R|L|-|C| |t|o| |c|a|n|c|e|l| @10|0|,|0|-|1| @8|A|l@1| 
+|W+0#0000000&|a|i|t|i|n|g| |f|o|r| |O|S|C|5|2| |r|e|s|p|o|n|s|e|.@2| |P|r|e|s@1| |C|T|R|L|-|C| |t|o| |c|a|n|c|e|l| @4|0|,|0|-|1| @8|A|l@1|