ixml parser: a whitespace-only value is content, not formatting

[oblomov-dev]

What is wrong

if_ixml_parser~parse skips the whitespace between two tags at the end of
every iteration, src/ixml/cl_ixml.clas.locals_imp.abap:

lv_rest = lv_xml.
SHIFT lv_rest LEFT DELETING LEADING lv_whitespace.
IF lv_rest IS INITIAL OR lv_rest(1) = '<'.
  lv_xml = lv_rest.
ENDIF.

That is right for the indentation of a pretty-printed document, but it also
fires when the whitespace is everything an element contains. <FIELD> </FIELD>
and <FIELD> + LF + </FIELD> have no text node afterwards, so the value is
gone.

Through CALL TRANSFORMATION id and the parse back this is a lost value — a
string holding nothing but blanks or a line break comes back initial:

DATA lv_xml TYPE string.
DATA: BEGIN OF ls_data,
        field TYPE string,
      END OF ls_data.

ls_data-field = ` `.

CALL TRANSFORMATION id SOURCE data = ls_data RESULT XML lv_xml.
" <FIELD> </FIELD>, correct
CLEAR ls_data.
CALL TRANSFORMATION id SOURCE XML lv_xml RESULT data = ls_data.

" system: ` `, here: `` - same for a value of one line feed

The change

The skip now keeps the whitespace when it stands directly between a start tag
and its own end tag, and drops it everywhere else:

IF lv_in_element = abap_true AND lv_rest CP '</*' AND lv_rest <> lv_xml.
  " content of this element, the value branch reads it
ELSE.
  lv_xml = lv_rest.
ENDIF.

lv_in_element is set where the tag is read: true after a start tag that is
not self-closing, false after the prolog, a close tag, a self-closing tag and
a value. Whitespace before a child element (<a> + LF + <b/>) or between
two siblings stays formatting and is dropped as before — the first child of a
pretty-printed element is still the element, not a #text node.

One behaviour does change on purpose: an element that contains only
whitespace now has a text node, <a> + LF + </a> reads as a line break.
That is what a system returns, and it is the same value the document was
written from.

Tests

• kernel_call_transformation: blank_only_roundtrip and
  newline_only_roundtrip — a value of one blank / one LF through
  CALL TRANSFORMATION id and back
• cl_ixml: parse_blank_only_value — <item> </item> keeps the blank
• cl_ixml: parse_indented_children — indentation still does not become a
  #text node

npm test is green: abaplint: 0 issue(s) found, 735 file(s) analyzed, and
the full unit run passes.

Why it matters downstream

abap2UI5 serializes the whole app state per request with
CALL TRANSFORMATION id into the draft and parses it back on the next one
(z2ui5_cl_ui5_util_context=>xml_stringify / xml_parse), so any value the
roundtrip drops is a value the app loses. After #1225 a line break inside a
value survives; a value that is a line break, or a blank, still does not.