users/21352: ensure $'' doesn't get elided.

Assign nulstring to it if empty.  Test for all forms of quotation
marks.

ChangeLog

@@ -1,5 +1,8 @@
 2016-03-07  Peter Stephenson  <p.stephenson@samsung.com>
 
+	* users/21352: Src/subst.c, Test/A03quoting.ztst: use nulstring
+	to ensure $'' isn't elided.
+
 	* 38094: Src/signals.c, Test/C03traps.ztst: POSIX exit traps
 	aren't local so local level should be 0 so they don't
 	get trashed if defined in a function.

Src/subst.c

@@ -145,8 +145,12 @@ stringsubstquote(char *strstart, char **pstrdpos)
 	    strret = dyncat(strstart, strsub);
     } else if (strdpos[len])
 	strret = dyncat(strsub, strdpos + len);
-    else
+    else if (*strsub)
 	strret = strsub;
+    else {
+	/* This ensures a $'' doesn't get elided. */
+	strret = dupstring(nulstring);
+    }
 
     *pstrdpos = strret + (strdpos - strstart) + strlen(strsub);
 

Test/A03quoting.ztst

@@ -74,3 +74,7 @@
 >16#61
 >16#62
 >16#64
+
+ () { print $# } '' "" $''
+0:$'' should not be elided, in common with other empty quotes
+>3