Cannot use `M-,` as `doom-localleader-alt-key`

### Observed behavior

Put this in `init.el`:
``` emacs-lisp
(define-key esc-map "," nil)
(setq doom-localleader-alt-key "M-,")
```
In insert mode, type `M-,`.

I get:
```
M-, is undefined
```

### Expected behavior

I would get the local leader menu.

### System information

- OS: gnu/linux (x86_64-pc-linux-gnu)
- Emacs: 26.2 (Apr 12, 2019)
- Doom: 2.0.9 (HEAD -> develop, github/develop c79cf3c8 2019-04-25 22:20:31 -0400)
- Graphic display: t (daemon: nil)
- System features: XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS GLIB NOTIFY ACL GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS LIBSYSTEMD LCMS2
- Details:
```elisp
env bootstrapper: envvar-file
elc count: 0
uname -a: Linux 5.0.9-arch1-1-ARCH #1 SMP PREEMPT Sat Apr 20 15:00:46 UTC 2019 x86_64
modules: (:completion company (ivy +icons) :ui doom doom-dashboard doom-quit hl-todo modeline nav-flash ophints (popup +all +defaults) treemacs vc-gutter vi-tilde-fringe window-select workspaces :editor evil file-templates fold multiple-cursors rotate-text snippets :emacs dired electric eshell imenu vc :tools eval flycheck flyspell (lookup +docsets) magit make password-store pdf :lang coq data emacs-lisp ess javascript latex ledger lua markdown ocaml (org +attach +babel +capture +export +habit +present +protocol) perl (sh +fish) web :app email (rss +org) :config literate (default +bindings +smartparens))
packages: (bbdb emojify org-review org-noter xah-replace-pairs org-ref (magit-todos :disable t) xah-math-input)
exec-path: (~/.opam/why3/bin ~/bin ~/.opam/why3/bin ~/bin /usr/local/bin /usr/bin /bin /usr/local/sbin /usr/lib/jvm/default/bin /usr/bin/site_perl /usr/bin/vendor_perl /usr/bin/core_perl /usr/lib/emacs/26.2/x86_64-pc-linux-gnu/)
```