Report details on why modification is a problem

When I execute acceptance tests in my provider, I get an error:

```
Error: Provider produced invalid plan

Provider "registry.terraform.io/hashicorp/buildx" planned an invalid value
for buildx_instance.foo.bootstrap: planned value cty.False for a non-computed
attribute.

This is a bug in the provider, which should be reported in the provider's own
issue tracker.
```

This might be related to my modifier implementation:

```
func (m boolDefaultModifier) Modify(ctx context.Context, req tfsdk.ModifyAttributePlanRequest, resp *tfsdk.ModifyAttributePlanResponse) {
b := req.AttributePlan.(types.Bool)

if !b.Null {
return
}

resp.AttributePlan = types.Bool{
Value: false,
}
}
```

Looking at the trace, all I see are messages like:

> attribute is not computed in schema, not marking unknown

So to me it seems like there is no real information as to what I am doing wrong - and I am pretty lost.
Would be great if there would be some useful trace or detailed error message.