[BUG]: github_organization_custom_properties: default_value of true_false properties reads back empty, causing a permanent plan diff

### Expected Behavior

A `github_organization_custom_properties` resource with `value_type = "true_false"` and `default_value = "false"` (or `"true"`) should plan clean after apply: GitHub stores the default, so Terraform should read it back unchanged.

### Actual Behavior

Every plan shows the default being (re-)added, forever:

```
# github_organization_custom_properties.this["fossa"] will be updated in-place
~ resource "github_organization_custom_properties" "this" {
+ default_value = "false"
id = "fossa"
# (6 unchanged attributes hidden)
}
```

GitHub has the value: `GET /orgs/{org}/properties/schema/fossa` returns `"default_value": "false"` and repositories report `"false"` for the property. The provider's Read uses `customProperty.DefaultValueString()`, which in go-github only returns a value for `string`, `single_select` and `url` types (it is documented as such); for `true_false` (and `multi_select`) it returns `("", false)`, so `default_value` is always written to state as `""`. The code carries a `// TODO: Add support for other types of default values` at that line.

A secondary issue in the same resource: Create/Update always sends `default_value` in the request body, as `""` when unset. GitHub does not accept an empty default for non-required properties, so `required = false` without a default cannot be applied reliably.

### Terraform Version

Terraform v1.14.9, provider integrations/github v6.13.0 (also present on `main`).

### GitHub Installation Type

GitHub.com (organization on the Team plan)

### Affected Resource(s)

- `github_organization_custom_properties`

### Terraform Configuration Files

```hcl
resource "github_organization_custom_properties" "fossa" {
property_name = "fossa"
value_type = "true_false"
required = true
default_value = "false"
description = "Require the fossa status check"
}
```

### Steps to Reproduce

1. `terraform apply` the configuration above (succeeds; the property is created with default `"false"`).
2. `terraform plan` → `+ default_value = "false"` is planned again. Repeat indefinitely.

### Debug Output

n/a — reproducible from the plan output above; root cause is visible in `resourceGithubCustomPropertiesRead` (`DefaultValueString()` for a `true_false` property).

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Start in resourceGithubCustomPropertiesRead, where DefaultValueString() is used, and inspect the Create/Update request handling for default_value. Reproduce the apply and plan cycle for a true_false property, then verify that a configured default remains in state and that an unset default does not send an empty value; the related resource tests, if present, should cover both cases.

Written by the indexing model from the issue text.