fix(glimmer): preserve empty text attributes

[tchak]

input

<button disabled></button>
<img alt="" />

before

<button disabled></button>
<img alt />

after

<button disabled></button>
<img alt="" />

[ikatyang]

FYI, glimmer parser is now available on the preview playground.

Prettier pr-4897
Playground link

--parser glimmer

Input:

<button disabled></button>
<img alt="" />

Output:

<button disabled></button><img alt="" />