try to avoid passing long xml strings to file.exists()

[JanMarvin]

Windows R-devels file.exists() does not like our approach of handing down potential XML or file path. I've added is_xml() in the if-clause. This should check if the xml character string is XML content and file.exists(xml) should be called only if xml is not XML.

Still, in this case we will have to parse actual XML strings twice, due to the nature of the is_xml() function.

Since this error is currently only in Windows R-Devel it might simply be something triggered by current R development. If this ERROR remains over the next couple of days or if CRAN reaches out, we still can merge this or come up with a better idea.

══ Failed tests ════════════════════════════════════════════════════════════════
  ── Error ('test-class-workbook.R:460'): add_drawing works ──────────────────────
  Error in `file.exists(xml)`: file name conversion problem -- name too long?
  Backtrace:
      ▆
   1. └─wb_workbook()$add_worksheet()$add_drawing(xml = tmp) at test-class-workbook.R:460:2
   2. └─openxlsx2::xml_node_name(xml)
   3. └─openxlsx2::read_xml(xml, ...)
   4. └─base::file.exists(xml)
  
  [ FAIL 1 | WARN 0 | SKIP 18 | PASS 1167 ]
  Error: Test failures
  Execution halted