Fix UTF8StringNormalize overrun on zero-length string

Detected by valgrind

servers/slapd/schema_init.c

@@ -1853,7 +1853,7 @@ UTF8StringNormalize(
 		}
 		nvalue.bv_val[nvalue.bv_len] = '\0';
 
-	} else {
+	} else if ( nvalue.bv_len )  {
 		/* string of all spaces is treated as one space */
 		nvalue.bv_val[0] = ' ';
 		nvalue.bv_val[1] = '\0';