minor fix for #annotate

Some github feedback pointed out that getting annotation input from
the player behaved differently from similar input for naming of
monsters and objects.  The complaint stated that hitting <return>
without supplying any input removed the old annotation, where other
naming would leave the old name intact.  3.6.0 did misbehave that
way; current code does too if EDIT_GETLIN is disabled but behaves
as desired when it's enabled.  (There's nothing that I can spot in
donamelevel() to explain why.  I'm confused.  Is tty_getlin()
returning the default answer instead of empty if that default text
is deleted at the prompt and no new text entered prior to <return>?)

Make donamelevel() work like mon/obj naming.  Empty input leaves
existing annotation, if any, intact.

doc/fixes36.1

@@ -537,6 +537,8 @@ gas spore explosion killing a gas spore which triggers a recursive explosion
 	they were both "gas spore's explosion" it wouldn't be noticeable (see
 	corresponding post-3.6.0 entry for more...)
 wizard mode 'sanity_check' gave spurious "mon not on map" warnings when mounted
+at the prompt for entering a level annotation, responding with <return>
+	erroneously removed old annotation; use <space><return> to do that
 
 
 Fixes to Post-3.6.0 Problems that Were Exposed Via git Repository