Save as html5 fails if there is text on a path

Migrated from https://bugs.launchpad.net/inkscape/+bug/1561443

Steps to reproduce:

• open Inkscape
• Create a path (e.g. straight line)
• Create some text
• Put text on path
• Save as HTML5

Example file: TextOnArcBug-20160324-01.svg

What happened?

No file saved, traceback:

/usr/lib/python3/dist-packages/apport/report.py:13: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
  import fnmatch, glob, traceback, errno, sys, atexit, locale, imp
Traceback (most recent call last):
  File "ink2canvas.py", line 72, in <module>
    Html5Canvas().run()
  File "/home/nal/all/inkscape/share/inkscape/extensions/inkex/base.py", line 123, in run
    self.save_raw(self.effect())
  File "/home/nal/all/inkscape/share/inkscape/extensions/inkex/base.py", line 146, in save_raw
    self.save(self.options.output)
  File "ink2canvas.py", line 36, in save
    self.walk_tree(svg_root, canvas)
  File "ink2canvas.py", line 67, in walk_tree
    self.walk_tree(node, canvas)
  File "ink2canvas.py", line 66, in walk_tree
    elem.draw()
  File "/home/nal/all/inkscape/share/inkscape/extensions/ink2canvas_lib/svg.py", line 293, in draw
    text = self.text_helper(tspan)
  File "/home/nal/all/inkscape/share/inkscape/extensions/ink2canvas_lib/svg.py", line 268, in text_helper
    return ts.text + self.text_helper(ts) + ts.tail
TypeError: must be str, not NoneType

In 0.92.x, traceback

Traceback (most recent call last):
  File "ink2canvas.py", line 91, in 
    ink.affect()
  File "/home/nal/all/ink92/share/inkscape/extensions/inkex.py", line 289, in affect
    self.effect()
  File "ink2canvas.py", line 86, in effect
    self.walk_tree(svg_root)
  File "ink2canvas.py", line 77, in walk_tree
    self.walk_tree(node)
  File "ink2canvas.py", line 76, in walk_tree
    elem.draw()
  File "/home/nal/all/ink92/share/inkscape/extensions/ink2canvas/svg.py", line 373, in draw
    text = self.text_helper(tspan)
  File "/home/nal/all/ink92/share/inkscape/extensions/ink2canvas/svg.py", line 348, in text_helper
    return ts.text + self.text_helper(ts) + ts.tail
TypeError: coercing to Unicode: need string or buffer, NoneType found

Version Info:

• Inkscape 1.1-dev (aa764a7b88, 2020-06-18, custom) with extensions@c26c0ec8 (master today) and the normal extension head, Linux Mint 19.1
• Also occurs in 0.92.5, but error message differs a bit
• Originally reportedin 0.91