style: use "-n" instead of "! -z" in shell scripts

[mboersma]

Reason for Change:

Enforces a more readable simplification of a common shell test.
See https://github.com/koalaman/shellcheck/wiki/SC2236

Issue Fixed:

Refs #953

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

[CecileRobertMichon]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CecileRobertMichon, mboersma

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [CecileRobertMichon,mboersma]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[codecov]

Codecov Report

Merging #1166 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1166   +/-   ##
=======================================
  Coverage   74.71%   74.71%           
=======================================
  Files         129      129           
  Lines       18385    18385           
=======================================
  Hits        13736    13736           
  Misses       3856     3856           
  Partials      793      793