don't panic with a null list block value in config

internal/plans/objchange/plan_valid.go

@@ -101,6 +101,14 @@ func assertPlanValid(schema *configschema.Block, priorState, config, plannedStat
 				continue
 			}
 
+			if configV.IsNull() {
+				// Configuration cannot decode a block into a null value, but
+				// we could be dealing with a null returned by a legacy
+				// provider and inserted via ignore_changes. Fix the value in
+				// place so the length can still be compared.
+				configV = cty.ListValEmpty(configV.Type().ElementType())
+			}
+
 			plannedL := plannedV.LengthInt()
 			configL := configV.LengthInt()
 			if plannedL != configL {

internal/plans/objchange/plan_valid_test.go

@@ -389,6 +389,41 @@ func TestAssertPlanValid(t *testing.T) {
 			},
 		},
 
+		// but don't panic on a null list just in case
+		"nested list, null in config": {
+			&configschema.Block{
+				BlockTypes: map[string]*configschema.NestedBlock{
+					"b": {
+						Nesting: configschema.NestingList,
+						Block: configschema.Block{
+							Attributes: map[string]*configschema.Attribute{
+								"c": {
+									Type:     cty.String,
+									Optional: true,
+								},
+							},
+						},
+					},
+				},
+			},
+			cty.ObjectVal(map[string]cty.Value{
+				"b": cty.ListValEmpty(cty.Object(map[string]cty.Type{
+					"c": cty.String,
+				})),
+			}),
+			cty.ObjectVal(map[string]cty.Value{
+				"b": cty.NullVal(cty.List(cty.Object(map[string]cty.Type{
+					"c": cty.String,
+				}))),
+			}),
+			cty.ObjectVal(map[string]cty.Value{
+				"b": cty.ListValEmpty(cty.Object(map[string]cty.Type{
+					"c": cty.String,
+				})),
+			}),
+			nil,
+		},
+
 		// blocks can be unknown when using dynamic
 		"nested list, unknown nested dynamic": {
 			&configschema.Block{
@@ -1671,7 +1706,7 @@ func TestAssertPlanValid(t *testing.T) {
 
 			t.Logf(
 				"\nprior:  %sconfig:  %splanned: %s",
-				dump.Value(test.Planned),
+				dump.Value(test.Prior),
 				dump.Value(test.Config),
 				dump.Value(test.Planned),
 			)