Fix panic due to missing configuration

[darkowlzz]

This adds a check for the provider resource data in all of the methods of bootstrap git resource that require the Git and Kubernetes configuration to run.

Missing configuration results in the following:

Plan: 1 to add, 0 to change, 0 to destroy.
╷
│ Error: Missing configuration
│ 
│   with flux_bootstrap_git.this,
│   on main.tf line 48, in resource "flux_bootstrap_git" "this":
│   48: resource "flux_bootstrap_git" "this" {
│ 
│ Git and Kubernetes configuration not found
╵

Fixes #508

[stefanprodan]

LGTM

Thanks @darkowlzz