Crash linking/embedding invalid file on import

Reported by @greytomorrow in #3560 (split out from it)

Steps to reproduce:

• create a empty file (call it "foo" or "foo.svg")
• open Inkscape
• file > import...
• select the empty file (may need to view All files instead of Inkscape files)
• Either Embed or Link the file (2nd or 3rd option)

What happened?

• Crash inkscape-backtrace-open-empty.txt

What should have happened?

• No crash
• In 0.92.x, these weren't options, Inkscape fails (error message, no crash) after selecting the empty file for import.

Version Info:

• 1.1-dev (c401a8e72c, 2020-11-05), 1.0alpha (9dee831, 2019-01-15) appimage Linux Mint 20
• 1.1-dev (7802ed9, 2020-09-04)/1.0(1.0+r73+1) macOS 10.13.6 (17G12034)/Ubuntu 18.04.4 LTS