fix(tooltip): don't run callbacks when the filter removes every item

[Minhal128]

Fixes #12274

The problem

Tooltip#update() decided whether the tooltip had anything to show by looking at this._active:

if (!active.length) {
  // hide
} else {
  tooltipItems = this._createItems(options);
  this.title = this.getTitle(tooltipItems, options);
  ...
}

options.filter is applied inside _createItems, after that branch is chosen. So when a filter rejects every active element there are still active elements, the visible branch runs, and getTitle / getBody / getFooter are invoked with an empty TooltipItem[].

That breaks callbacks which reasonably assume at least one item — as reported, users have to add a guard to their title callback that is unnecessary in every other situation.

The change

Build the items first, then branch on the filtered result. A tooltip with nothing left to show is now treated the same as one with no active elements: callbacks are skipped and it stays hidden.

_createItems is still called exactly once per update, and is still skipped entirely when there are no active elements, so dataPoints / labelColors keep their existing lifecycle.

Verification

Added a regression test that filters out every item and asserts the title callback is never invoked and the tooltip stays at opacity: 0.

I ran the full CI suite on a fork before opening this (workflow_dispatch on the branch), and the same suite on unmodified master as a baseline:

	tests	result
master (baseline)	1703	1 failed — controller.polarArea/pointLabels/overlapping.js, Firefox
this branch	1704	same 1 failure, nothing else

That single failure is a pre-existing pixel fixture on Firefox and is present without this change. Windows is fully green on both.

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