Fix some whitespace issues in XMLSERIALIZE(... INDENT).

Fix some whitespace issues in XMLSERIALIZE(... INDENT).

We must drop whitespace while parsing the input, else libxml2
will include "blank" nodes that interfere with the desired
indentation behavior.  The end result is that we didn't indent
nodes separated by whitespace.

Also, it seems that libxml2 may add a trailing newline when working
in DOCUMENT mode.  This is semantically insignificant, so strip it.

This is in the gray area between being a bug fix and a definition
change.  However, the INDENT option is still pretty new (since v16),
so I think we can get away with changing this in stable branches.
Hence, back-patch to v16.

Jim Jones

Discussion: https://postgr.es/m/872865a8-548b-48e1-bfcd-4e38e672c1e4@uni-muenster.de