Reduce page_props usage

Don't save page_top_level_section_count=0,
treat the lack of property as zero instead.
That would reduce page_props usage by 23% on enwiki, or 6.5M rows.

Note: Although lack of property could also mean the talk page count
hasn't been computed this code has been on enwiki for some time
let's not worry too much about this edgecase.

Change-Id: Ia378290703dc4266d9b61a6aab7d2b002c3e740d
Merge "Reduce page_props usage"