Parts update improvement

[nilmerg]

Adds only some minor fixes. I noticed a repeated pattern with usages of TermInput::prepareMultipartUpdate():

foreach ($termInput->prepareMultipartUpdate() as $update) {
    if (! is_array($update)) {
        $update = [$update];
    }

    $controller->addPart(...$update);
}

It can now be reduced to this:

foreach ($termInput->prepareMultipartUpdate() as $update) {
    $controller->addPart(...$update);
}

[Jan-Schuppik]

Looks good to me