Fix stacked area hide show bug

[emilykl]

Closes #7660

Fixes issue where some traces in stacked area charts were not rendering their fill correctly after certain sequences of hide/show operations in the legend.

Steps to test

Reproduce existing issue

1. Go to this Codepen (from the docs)
2. Follow the exact reproduction steps described in 7660:

• Double-click "trace 1" to show "trace 1" only
• Click "trace 1" again
• Now plot is empty
• Double click any trace to show all traces again

3. The orange trace should be missing a fill. This is the bug.

Confirm that this PR fixes the issue

1. In the Codepen, replace the plotly.js script src with the build for this branch from the CI: https://output.circle-artifacts.com/output/job/c4caeb45-0a0b-4e23-97b6-ebb26b8561c8/artifacts/0/dist/plotly.js
2. Repeat the steps under (2) above
3. All trace fills should render normally.

[camdecoster]

Since you're making _ownFill and _nextFill null, the if checks will always fail. So, let's delete them.

Suggested change

	// Clear any existing fill element references to ensure clean state
	// This prevents stale references when fill configuration changes
	trace._ownFill = null;
	trace._nextFill = null;
	var fillData = [];
	if(trace._ownfill) fillData.push('_ownFill');
	if(trace._nexttrace) fillData.push('_nextFill');
	// Clear any existing fill element references to ensure clean state
	// This prevents stale references when fill configuration changes
	trace._ownFill = null;
	trace._nextFill = null;
	var fillData = [];

[emilykl]

@camdecoster Tragically, _ownFill and _ownfill, as well as _nextFill and _nexttrace, are separate variables. 🙃

[camdecoster]

Ah, fair enough. I have words about this, but I won't include them here publicly.