Ensure Dataset#page_count in pagination extension is at least one (Fixes #714)

This breaks backwards compatibility, but the previous behavior
didn't make sense.  For an empty dataset, there should be a single
page with no records.  By making page_count = 1 for an empty
dataset, this fixes last_page?, page_range, and potentially other
methods when the dataset is empty.
Fix literalization of empty blobs on MySQL (Fixes #715)

MySQL doesn't like empty blobs when hex escaped, so use an empty
string in that case.