Avoid a partially nil host as sink in test muxed providers

This change prevents test providers from being initialized with partially nil logging
sinks, which panicked in tests. Happily, the problem is restricted to `MakeMuxedServer`, a
testing API. This was never present in production code.

Fixes #2697

---

This is [my least favorite go feature](https://www.iwahbe.com/blog/go-pointers-considered-confusing/).

pkg/pf/tfbridge/main.go

@@ -27,6 +27,7 @@ import (
 	"github.com/pulumi/pulumi/sdk/v3/go/common/util/contract"
 	pulumirpc "github.com/pulumi/pulumi/sdk/v3/proto/go"
 
+	"github.com/pulumi/pulumi-terraform-bridge/v3/internal/logging"
 	"github.com/pulumi/pulumi-terraform-bridge/v3/pkg/pf"
 	pfmuxer "github.com/pulumi/pulumi-terraform-bridge/v3/pkg/pf/internal/muxer"
 	"github.com/pulumi/pulumi-terraform-bridge/v3/pkg/tfbridge"
@@ -126,7 +127,7 @@ func MainWithMuxer(ctx context.Context, pkg string, info tfbridge.ProviderInfo,
 // of pulumi-terraform-bridge.  This is an experimental API.
 func MakeMuxedServer(
 	ctx context.Context, pkg string, info tfbridge.ProviderInfo, schema []byte,
-) func(host *rprovider.HostClient) (pulumirpc.ResourceProviderServer, error) {
+) func(*rprovider.HostClient) (pulumirpc.ResourceProviderServer, error) {
 	shim, ok := info.P.(*pfmuxer.ProviderShim)
 	contract.Assertf(ok, "MainWithMuxer must have a ProviderInfo.P created with AugmentShimWithPF")
 	_, err := shim.ResolveDispatch(&info)
@@ -172,7 +173,15 @@ func MakeMuxedServer(
 					Server: func(host *rprovider.HostClient) (pulumirpc.ResourceProviderServer, error) {
 						infoCopy := info
 						infoCopy.P = prov
-						return NewProviderServer(ctx, host,
+
+						// https://github.com/pulumi/pulumi-terraform-bridge/issues/2697:
+						// Avoid accidentally casting (*rprovider.HostClient)(nil) to
+						// (logging.Sink)((*rprovider.HostClient)(nil)).
+						var sink logging.Sink
+						if host != nil {
+							sink = host
+						}
+						return NewProviderServer(ctx, sink,
 							infoCopy, ProviderMetadata{PackageSchema: schema})
 					},
 				})