Only insert XML declarations in nxml-mode when the buffer is empty

Only insert XML declarations in nxml-mode when the buffer is empty

* lisp/nxml/nxml-mode.el (nxml-mode): Don't insert the XML
declaration unless it's an empty buffer (bug#24978).  This avoids
the problem of the declaration being inserted by mistake when
opening XML files from archives and the like -- the file doesn't
exist on the file system there, either, so it would typically lead
to doubled XML declarations.