Fix #69181: READ_CSV|DROP_NEW_LINE drops newlines within fields

One may argue that `DROP_NEW_LINE` does not make sense in combination
with `READ_CSV`, but without `DROP_NEW_LINE`, `SKIP_

Fix #69181: READ_CSV|DROP_NEW_LINE drops newlines within fields

One may argue that `DROP_NEW_LINE` does not make sense in combination
with `READ_CSV`, but without `DROP_NEW_LINE`, `SKIP_EMPTY` does not
skip empty lines at all. We could fix that, but do not for BC reasons.
Instead we no longer drop newlines in `spl_filesystem_file_read_ex()`
when reading CSV, but handle that in `spl_filesystem_file_read_csv()`
by treating lines with only (CR)LF as being empty as well.

Closes GH-7618.

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