[SC3028] In a if [ -n BASH_SOURCE ] block don't warn of BASH_SOURCE being undefined

[SamuelMarks]

For bugs

• Rule Id (if any, e.g. SC1000): SC3028
• My shellcheck version (shellcheck --version or "online"): 0.10.0
• The rule's wiki page does not already cover this (e.g. https://shellcheck.net/wiki/SC2086)
• I tried on https://www.shellcheck.net/ and verified that this is still a problem on the latest commit

For new checks and feature suggestions

• https://www.shellcheck.net/ (i.e. the latest commit) currently gives no useful warnings about this
• I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

Here's a snippet or screenshot that shows the problem:

#!/bin/sh

if [ -n "${BASH_VERSION}" ]; then
  # shellcheck disable=SC3028 disable=SC3054
  this_file="${BASH_SOURCE[0]}"
elif [ -n "${ZSH_VERSION}" ]; then
  # shellcheck disable=SC2296
  this_file="${(%):-%x}"
else
  this_file="${0}"
fi

Here's what shellcheck currently says:

When I remove the comments it says:

In POSIX sh, BASH_SOURCE is undefined.
See SC3028.
In POSIX sh, array references are undefined.
See SC3054.

Here's what I wanted or expected to see:

No error

FYI: Similar errors occur with pipefail:

In POSIX sh, set option pipefail is undefined.
See SC3040.