HTML output can add empty tables for stat lines

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If a game system (e.g. Warhammer 40,000) has multiple stat lines then the HTML output writes a table per stat line format instead of trying to mash them all in to one table.

If a game system has multiple stat lines but the user doesn't take any units with one of those stat lines then the exporter still writes out the table headers. This is unnecessary and should be avoided.

[ibboard]

(In r291) Fixes #174: HTML output can add empty tables for stat lines

• Check if table has rows before outputting it

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