Don't report IAccessible alert events if there's no content.

[jcsteh]

Link to issue number:

Fixes #5657.

Summary of the issue:

In Firefox and Chrome, NVDA reports "alert" when an empty element with role="alert" is shown.

Description of how this pull request fixes the issue:

In IAccessible's event_alert, return early if there's no content.
As well as the childCount, we also check name and description, as these might be set using ARIA even on an alert with no children.

Testing performed:

In both Firefox and Chrome, tested with the following test cases.

Empty alert. When button is pressed, nothing is reported:
data:text/html,<div id="al" role="alert" hidden></div><button onClick="al.hidden = false;">Alert</button>

Alert with text content. When button is pressed, "alert text" is reported:
data:text/html,<div id="al" role="alert" hidden>text</div><button onClick="al.hidden = false;">Alert</button>

Alert with aria-label. When button is pressed, "label alert" is reported:
data:text/html,<div id="al" role="alert" aria-label="label" hidden></div><button onClick="al.hidden = false;">Alert</button>

Alert with aria-describedby. When button is pressed, "alert desc" is reported:
data:text/html,<div id="desc">desc</div><div id="al" role="alert" aria-describedby="desc" hidden></div><button onClick="al.hidden = false;">Alert</button>

Alert with button. When button outside the alert is pressed, "alert, inner button" is reported:
data:text/html,<div id="al" role="alert" hidden><button>inner</button></div><button onClick="al.hidden = false;">Alert</button>

Known issues with pull request:

There are obscure cases this won't catch like an alert containing an empty paragraph, but fixing that is somewhat trickier and I doubt it happens in the real world. If it does, we can extend this later.

Change log entry:

Bug Fixes:

- In Mozilla Firefox and Google Chrome, empty alerts are no longer reported.