skip query cache if it's not set

[eRadical]

Template query cache values to be skipped in case they're not provided.

Need to keep them templated as MySQL & MariaDB diverged in behavior (MySQL>=5.7.20 removed QC, MariaDB enabled it by default).

Related Issue

Closes #184

Types of changes

• 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)
• Chore (non-breaking change that does not add functionality or fix an issue)

Checklist:

• I have read the CONTRIBUTING document.
• I have run the pre-merge tests locally and they pass.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.