Allow 0-width cells

The presence of a content-less cell would causes warnings:

    Use of uninitialized value in subroutine entry
      at [...]/HTML/HTML5/Table/Col.pm line 74

Additionally, if a column contains no cells with content, rendering
aborts:

    Attribute (width) does not pass the type constraint because:
      Validation failed for 'Num' with value undef

Prevent both of these errors by ensuring that needs_width on each cell
returns a 0 in the event that it is content-less.
Don't abort on tables with no columns

Due to _build_height returning undef if there were no cells, rendering
of a table with no <td>s would abort:

    Attribute (height) does not pass the type constraint because:
      Validation failed for 'Num' with value undef

Ensure that _build_height always returns a defined value.