extend the ZLE_RPROMPT_INDENT=0 workaround to a few more cases

If the last right prompt line can be proven to always have
zero length after prompt expansion, we can unset RPROMPT
thus avoiding triggering zsh bugs related to ZLE_RPROMPT_INDENT=0.

Fixes #458.

internal/p10k.zsh

@@ -4752,6 +4752,8 @@ function _p9k_set_prompt() {
   _p9k_prompt_side=
   (( $#_p9k_cache < _POWERLEVEL9K_MAX_CACHE_SIZE )) || _p9k_cache=()
   (( $#_p9k__cache_ephemeral < _POWERLEVEL9K_MAX_CACHE_SIZE )) || _p9k__cache_ephemeral=()
+
+  [[ -n $RPROMPT ]] || unset RPROMPT
 }
 
 _p9k_set_instant_prompt() {
@@ -4761,6 +4763,7 @@ _p9k_set_instant_prompt() {
   typeset -g _p9k_instant_prompt=$PROMPT$'\x1f'$_p9k__prompt$'\x1f'$RPROMPT
   PROMPT=$saved_prompt
   RPROMPT=$saved_rprompt
+  [[ -n $RPROMPT ]] || unset RPROMPT
 }
 
 typeset -gri __p9k_instant_prompt_version=18
@@ -6327,6 +6330,13 @@ _p9k_init_lines() {
     [[ -n $_p9k_ret ]] && _p9k_line_never_empty_right+=1 || _p9k_line_never_empty_right+=0
     _p9k_line_prefix_right+='${_p9k__'$i'r-${${:-${_p9k_bg::=NONE}${_p9k_i::=0}${_p9k_sss::='$_p9k_ret'}}+}'
     _p9k_line_suffix_right+='$_p9k_sss%b%k%f}'  # gets overridden for _p9k_emulate_zero_rprompt_indent
+    if (( i == num_lines )); then
+      typeset -p i num_lines _p9k_ret
+      # it's safe to use _p9k_prompt_length on the last line because it cannot have prompt connection
+      _p9k_prompt_length ${(e)_p9k_ret}
+      typeset -p i num_lines _p9k_ret
+      (( _p9k_ret )) || _p9k_line_never_empty_right[-1]=0
+    fi
   done
 
   _p9k_get_icon '' LEFT_SEGMENT_END_SEPARATOR
@@ -6380,7 +6390,9 @@ _p9k_init_lines() {
     if [[ -n $_p9k_ret ]]; then
       [[ _p9k_ret == *%* ]] && _p9k_ret+=%b%k%f
       _p9k_line_suffix_right[-1]+='${_p9k__'$num_lines'r_frame-'${(qqq)_p9k_ret}'}'
-      _p9k_line_never_empty_right[-1]=1
+      # it's safe to use _p9k_prompt_length on the last line because it cannot have prompt connection
+      _p9k_prompt_length $_p9k_ret
+      (( _p9k_ret )) && _p9k_line_never_empty_right[-1]=1
     fi
 
     if (( num_lines > 2 )); then