Fix Tables schema for empty layers (#441) #674

Job	Run time
Documentation	4m 57s
Julia lts - ubuntu-latest - x64 - push	4m 18s
Julia lts - macOS-latest - x64 - push	5m 37s
Julia lts - windows-latest - x64 - push	7m 22s
Julia 1 - ubuntu-latest - x64 - push	5m 50s
Julia 1 - macOS-latest - x64 - push	8m 26s
Julia 1 - windows-latest - x64 - push	9m 6s
Julia nightly - ubuntu-latest - x64 - push	6m 7s
Julia nightly - macOS-latest - x64 - push	5m 17s
Julia nightly - windows-latest - x64 - push	10m 10s
	1h 7m 10s

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