[XXE] Nonbreaking space in head_row_column template

[maxwell]

The DB5 template for head_row_column (in the file
addon/config/docbook5/docbook5_support.incl) automatically inserts a
non-breaking space char (&#160; = U+00A0) in the <entry> elements of the
header row.  This appears to be intentional, but we can't figure out why. 
Is it because these <entry> elements have the attr 'align="center"', and
the style sheets don't know how to handle that attr if there isn't anything
to center?  (They're also in the templates for <informaltable>, and both
for the "head_row" styles and the "head_row_column" styles.)

   Mike Maxwell