man2html: Handle table format spec which consists of only '_'.

man2html: Handle table format spec which consists of only '_'.

Before this commit, a data line corresponding to the table
format specification which consists of only '_' is disappeared
during conversion by man2html. For example, in signal(7)
a first row next to the title disappears after HTML conversion.

This commit adds a function to check if a table format spec for
a current row has one or more data item and during scanning data
items scan_table() skips such a table format spec without data items.