A submit, reset or JS button with no name warns: Form.php reads objattr["fieldname"] unguarded

[jakejackson1]

Form::print_ob_button() reads
$objattr['fieldname'] without checking it is there:

} elseif ($objattr['subtype'] === 'SUBMIT') {
	$url = $this->formAction;
	$type = $this->formExportType;
	$method = $this->formMethod;
	$this->SetFormButtonText($objattr['value']);
	$this->SetFormSubmit($w, $h, $objattr['fieldname'], ...);

Form.php:532, with the same read at :526 (RESET) and :540 (BUTTON). The neighbouring
optional attributes on the same lines - background-col, border-col, noprint - are all
isset()-guarded, so the shape is deliberate everywhere except this one.

Tag\Input only sets fieldname when the element carries a name, and the HTML spec does
not require one on a submit button: a submit button with no name is not submitted, which is
the ordinary way to write a button that only triggers the form.

Reproducing

$mpdf = new Mpdf();
$mpdf->useActiveForms = true;
$mpdf->WriteHTML('<form><input type="submit" value="Go"></form>');
$mpdf->Output('', 'S');

Warning: Undefined array key "fieldname" in src/Form.php on line 532

Adding name="go" silences it.

Expected: a submit button with no name is given whatever SetFormSubmit() uses for an unnamed
field, the way the optional colours beside it are handled. Observed: a warning under PHP 8, a
notice under PHP 7, and null passed as the field name.

Found while fixing #141; not that issue's shape and deliberately left out of that PR.

Measured on 2982df6, PHP 8.5.4.