Remove xmlparse(document '') test

This one test was behaving differently between the ubuntu fix for
CVE-2015-7499 and the base "expected" file.  It's not worth having
yet another version of the expected file for this test, so drop it.
Perhaps at some point when all distros have settled down to the
same behavior on this test, it can be restored.

Problem found by me on libxml2 (2.9.1+dfsg1-3ubuntu4.6).
Solution suggested by Tom Lane.
Backpatch to 9.5, where the test was added.

src/test/regress/expected/xml.out

@@ -263,17 +263,6 @@ SELECT xmlparse(content '<nosuchprefix:tag/>');
  <nosuchprefix:tag/>
 (1 row)
 
-SELECT xmlparse(document '');
-ERROR:  invalid XML document
-DETAIL:  line 1: switching encoding : no input
-
-^
-line 1: Document is empty
-
-^
-line 1: Start tag expected, '<' not found
-
-^
 SELECT xmlparse(document '   ');
 ERROR:  invalid XML document
 DETAIL:  line 1: Start tag expected, '<' not found

src/test/regress/expected/xml_1.out

@@ -204,10 +204,6 @@ SELECT xmlparse(content '<nosuchprefix:tag/>');
 ERROR:  unsupported XML feature
 DETAIL:  This functionality requires the server to be built with libxml support.
 HINT:  You need to rebuild PostgreSQL using --with-libxml.
-SELECT xmlparse(document '');
-ERROR:  unsupported XML feature
-DETAIL:  This functionality requires the server to be built with libxml support.
-HINT:  You need to rebuild PostgreSQL using --with-libxml.
 SELECT xmlparse(document '   ');
 ERROR:  unsupported XML feature
 DETAIL:  This functionality requires the server to be built with libxml support.

src/test/regress/expected/xml_2.out

@@ -251,10 +251,6 @@ SELECT xmlparse(content '<nosuchprefix:tag/>');
  <nosuchprefix:tag/>
 (1 row)
 
-SELECT xmlparse(document '');
-ERROR:  invalid XML document
-DETAIL:  line 1: switching encoding : no input
-line 1: Document is empty
 SELECT xmlparse(document '   ');
 ERROR:  invalid XML document
 DETAIL:  line 1: Start tag expected, '<' not found

src/test/regress/sql/xml.sql

@@ -71,7 +71,6 @@ SELECT xmlparse(content '<relativens xmlns=''relative''/>');
 SELECT xmlparse(content '<twoerrors>&idontexist;</unbalanced>');
 SELECT xmlparse(content '<nosuchprefix:tag/>');
 
-SELECT xmlparse(document '');
 SELECT xmlparse(document '   ');
 SELECT xmlparse(document 'abc');
 SELECT xmlparse(document '<abc>x</abc>');