fix: Table.HeaderCell renders as a td when it has no cell content

[kurtdoherty]

What it says on the tin.

Header cells are not permitted to have empty content: https://dequeuniversity.com/rules/axe/4.8/empty-table-header?application=axeAPI

[rpt-uk-github]

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[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 23987571	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (2398757)	Report Missing	Report Missing	Report Missing
Head commit (abbec7a)	8636	7997	92.60%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#735)	2	2	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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