bug#31344: 26.1; ispell enchant support generates invalid regexp for empty extra chars

[Daniel Pittman <slippycheeze@google.com>]

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G'day.  With the new Enchant support in 26 I was motivated to try it out.
Turns out that some languages generate an empty "extra characters" report
for languages I use, which is turned into the (invalid, says `looking-at`)
regular expression "[]".

This triggers in this function in ispell.el:

(defun ispell--get-extra-word-characters (&optional lang)
   "Get the extra word characters for LANG as a character class.
If LANG is omitted, get the extra word characters for the default language."
   (concat "[" (string-trim-right (apply 'ispell--call-enchant-lsmod
                                         (append '("-word-chars") (if lang
`(,lang))))) "]"))

Obviously, when the result of that call is an empty string we get `[]`; in
my case:

] enchant-2-lsmod -word-chars en_US | xxd
00000000: 0a                                       .

...which causes that.  (Error is: `(invalid-regexp "Unmatched [ or [^")`)

I have attached a patch which correctly handles this case, though I feel
there must be a cleaner way to handle this modification of the return.

In any case, Google (and I personally) have papers on file with the FSF
covering this contribution, if it is non-trivial enough for that to matter.

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--- ispell.el.orig	2018-05-02 15:58:31.000000000 -0400
+++ ispell.el	2018-05-02 16:03:49.000000000 -0400
@@ -1206,8 +1206,12 @@
 (defun ispell--get-extra-word-characters (&optional lang)
   "Get the extra word characters for LANG as a character class.
 If LANG is omitted, get the extra word characters for the default language."
-  (concat "[" (string-trim-right (apply 'ispell--call-enchant-lsmod
-                                        (append '("-word-chars") (if lang `(,lang))))) "]"))
+  (let ((extra (string-trim-right
+                (apply 'ispell--call-enchant-lsmod
+                       (append '("-word-chars") (if lang `(,lang)))))))
+    (if (equal extra "")
+        ""
+      (concat "[" extra "]"))))
 
 (defun ispell-find-enchant-dictionaries ()
   "Find Enchant's dictionaries, and record in `ispell-enchant-dictionary-alist'."