avoid errors unbound variables and unset parameters

[LucasLarson]

Description

zsh prints an abundance of errors when setopt nounset is enabled and fast-syntax-highlighting is activated. This commit helps to avoid the unnecessary verbosity and fix #49. cases where variables were referenced using $var and where zsh5.9 --no-unset complained about it have been modified to use ${var-} syntax

Related Issue

49

How Has This Been Tested?

I tested my changes manually using my default login shell (zsh -u at the moment) with my changes to my local copy of the plugin.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[pschmitt]

Lgtm.