fix #9042, #7138: handle empty placeholder case in IE

[nciont]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:
Before this fix, the workaround for #9042 was to provide null or false value instead of empty string: :placeholder="placeholder || null".
The fix from #7138 was incomplete as it did not account for the case when placeholder attribute exists but is empty and the bug is not triggered in IE.
A new test case ensures the first triggered event does not get blocked.

[yyx990803]

Thanks for the fix! When investigating this I also noticed that the bug actually only applies to <textarea> and not <input>... so this should fix #9383 as well