fix(image): honor file path when saving WebP images

[joshtrichards]

Summary

Fix WebP image saving by passing the requested destination path to imagewebp().

Previously, the WebP branch in OC\Image::_output() passed null as the target path, which could write the image to output instead of saving it to the requested file.

Noticed this in Image::_output() while perusing the code. While it's almost certainly a bug that breaks save(), admittedly I also can't find any callers of save()... but it is part of our public API so at least correcting this as long as the function still exists...

server/lib/public/IImage.php

Lines 68 to 75 in ba137f0

	/**
	* Saves the image.
	*
	* @param string $filePath
	* @param string $mimeType
	* @since 8.1.0
	*/
	public function save(?string $filePath = null, ?string $mimeType = null): bool;

Tests pass for save() because it only impacts WebP image format specifically, which isn't a format specifically covered.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[joshtrichards]

May fold into #61637 to avoid rebasing/conflicts