Clarify what it means for the system identifier to be missing

Currently in section 8.2.4.1 The initial phase it speaks of when the  
system identifier is missing, even though from the parser there will  
always be a system identifier, even if it has a zero length. It should  
be spoken of as being empty (i.e., zero length) and not missing.

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Geoffrey Sneddon
<http://gsnedders.com/>