Fix issue when using ./configure --with-readline.

I delved through the readline docs and code, and couldn't
see a configuration for this.
I.e. it seems implicit to readline to not echo empty lines
when a prompt is not displayed.

The attached adjusts bc to be consistent in this regard
with other line input methods.

Empty lines are very useful when using bc interactively,
to delimit one's work

diff -Naur bc-1.07.1/bc/scan.l bc-1.07.1-new/bc/scan.l
--- bc-1.07.1/bc/scan.l	2017-04-07 22:20:02.000000000 +0000
+++ bc-1.07.1-new/bc/scan.l	2021-08-14 12:12:33.860991777 +0000
@@ -170,6 +170,10 @@
       if (rl_len != 1)
 	add_history (rl_line); 
       rl_line[rl_len-1] = '\n';
+      /* readline doesn't echo empty lines without a prompt,
+         so do so here. */
+      if (rl_len == 1)
+        putchar ('\n');
       fflush (stdout);
     }