Remove ambiguousness from return value of `is_textdomain_loaded()`

[jrf]

The is_textdomain_loaded() function - as per the doc block - is supposed to answer the question whether there are any translations loaded for a textdomain.

It currently does not do so.

It only checks whether the $i10n['domain'] key isset, not whether there are any translations.

As the first time the get_translations_for_domain() function is called, it will also set the text domain, this can lead to false positives, i.e. is_textdomain_loaded() reporting true for a textdomain which hasn't been loaded.

Simple example to reproduce the issue:

<?php
var_dump( is_textdomain_loaded( 'does-not-exist' ) );
echo __( 'just some string', 'does-not-exist' );
var_dump( is_textdomain_loaded( 'does-not-exist' ) );

will currently result in:

false
just some string
true

With the patch the result will be:

false
just some string
false

This patch will improve the results for patch #35439 as well.

[jrf]

Example code as unit test for this issue

[ocean90]

Duplicate of #21319.

[jrf]

@ocean90 your search was obviously better than mine ;-)
I considered changing the get_translations_for_domain() function as well like in the other issue, but felt that the risk of breaking things which expected an object would be too big.