reader: Only maintain cursor position in non-empty prefix search

Otherwise this would always move the cursor to the beginning.

Fixes #11133

src/reader.rs

@@ -1671,7 +1671,7 @@ fn update_command_line_from_history_search(&mut self) {
                 0..self.command_line.len(),
                 new_text,
             );
-            if self.history_search.by_prefix() {
+            if self.history_search.by_prefix() && !self.history_search.search_string().is_empty() {
                 self.command_line
                     .set_position(self.history_search.search_string().len());
             }

tests/pexpects/bind.py

@@ -364,6 +364,14 @@ send('\x02\x02\x02') # ctrl-b, backward-char
 sendline('\x1bu') # alt+u, upcase word
 expect_prompt("fooBAR")
 
+sendline('bind ctrl-z history-prefix-search-backward')
+expect_prompt()
+sendline("echo this continues")
+expect_prompt()
+send("\x1A")
+sendline(" with this text")
+expect_prompt("this continues with this text")
+
 sendline("""
     bind ctrl-g "
         commandline --insert 'echo foo ar'