fix: replace Vault`isPluginEnabled` check with null check as plugin not necessarily enabled yet

[Test-Account666]

Overview

In some cases, the isPluginEnabled check will cause the NullEconHandler to be used - Even though everything will work out.

Description

I noticed that PlotSquared will use the NullEconHandler in some scenarios. One of them is when the plugin providing the Economy is depending on Vault and PlotSquared.

I fixed this by replacing the Vault isPluginEnabled check with a null check.

The VaultPermissionHandler is also affected, although I don't know what's causing the issue here. I did not replace the isPluginEnabled this time, as it breaks permissions otherwise (See previous pull request).

Note that I kept the Vault isPluginEnabled check in the ServerListener, since Vault is going to be enabled at this point.

### Submitter Checklist
- [x] Make sure you are opening from a topic branch (**/feature/fix/docs/ branch** (right side)) and not your main branch.
- [x] Ensure that the pull request title represents the desired changelog entry.
- [x] New public fields and methods are annotated with `@since TODO`.
- [x] I read and followed the [contribution guidelines](https://github.com/IntellectualSites/.github/blob/main/CONTRIBUTING.md).