Bug when using --insert without --no-backup

[zeitounator]

When using the --insert option to write the TOC back to the analyzed file without using the --no-backup option, there is a bug because the no_backup variable is not set in the environment.

Example:

$ gh-md-toc --insert some_file.md 

Table of Contents
=================

   * [generated toc cut off in this example](#generated-toc-cut-off-in-this-example)
Found markers

/home/user/bin/gh-md-toc: line 180: [: =: unary operator expected
!! TOC was added into: 'some_file.md'
!! Origin version of the file: 'some_file.md.orig.2021-01-30_142030'
!! TOC added into a separate file: 'some_file.md.toc.2021-01-30_142030'

As you can see above, there is an error message about a unary operator expected on line line 180. This is because the variable is not secured with quotes for the comparison and leads to an error if not set.

This PR fixes this issue (plus secures an other line to prevent further equivalent bugs).

[ekalinin]

Thanks!