don't replace null strings with empty strings

This adds unexpected values in some cases, and since the case this
handles is only within set objects, we'll deal woth this when tackling
the sets themselves.

helper/plugin/grpc_provider.go

@@ -1119,13 +1119,6 @@ func stripSchema(s *schema.Schema) *schema.Schema {
 func copyMissingValues(dst, src cty.Value) cty.Value {
 	ty := dst.Type()
 
-	// In this case the provider set an empty string which was lost in
-	// conversion. Since src is unknown, there must have been a corresponding
-	// value set.
-	if ty == cty.String && dst.IsNull() && !src.IsKnown() {
-		return cty.StringVal("")
-	}
-
 	if src.IsNull() || !src.IsKnown() || !dst.IsKnown() {
 		return dst
 	}

helper/plugin/grpc_provider_test.go

@@ -821,27 +821,6 @@ func TestCopyMissingValues(t *testing.T) {
 				}),
 			}),
 		},
-		{
-			// Recover the lost unknown key, assuming it was set to an empty
-			// string and lost.
-			Src: cty.ObjectVal(map[string]cty.Value{
-				"map": cty.MapVal(map[string]cty.Value{
-					"a": cty.StringVal("a"),
-					"b": cty.UnknownVal(cty.String),
-				}),
-			}),
-			Dst: cty.ObjectVal(map[string]cty.Value{
-				"map": cty.MapVal(map[string]cty.Value{
-					"a": cty.StringVal("a"),
-				}),
-			}),
-			Expect: cty.ObjectVal(map[string]cty.Value{
-				"map": cty.MapVal(map[string]cty.Value{
-					"a": cty.StringVal("a"),
-					"b": cty.StringVal(""),
-				}),
-			}),
-		},
 	} {
 		t.Run(fmt.Sprintf("%d", i), func(t *testing.T) {
 			got := copyMissingValues(tc.Dst, tc.Src)