return a NullVal when presented with a nil flatmap

This allows us to decode the cty.Value back to a nil map, rather than
trying to create a map with nil values.

config/hcl2shim/flatmap.go

@@ -131,6 +131,9 @@ func flatmapValueFromHCL2Seq(m map[string]string, prefix string, val cty.Value)
 // The result may contain null values if the given map does not contain keys
 // for all of the different key paths implied by the given type.
 func HCL2ValueFromFlatmap(m map[string]string, ty cty.Type) (cty.Value, error) {
+	if m == nil {
+		return cty.NullVal(ty), nil
+	}
 	if !ty.IsObjectType() {
 		panic(fmt.Sprintf("HCL2ValueFromFlatmap called on %#v", ty))
 	}

config/hcl2shim/flatmap_test.go

@@ -522,6 +522,19 @@ func TestHCL2ValueFromFlatmap(t *testing.T) {
 			}),
 			WantErr: `invalid count value for "foo." in state: strconv.Atoi: parsing "not-valid": invalid syntax`,
 		},
+		{
+			Flatmap: nil,
+			Type: cty.Object(map[string]cty.Type{
+				"foo": cty.Set(cty.Object(map[string]cty.Type{
+					"bar": cty.String,
+				})),
+			}),
+			Want: cty.NullVal(cty.Object(map[string]cty.Type{
+				"foo": cty.Set(cty.Object(map[string]cty.Type{
+					"bar": cty.String,
+				})),
+			})),
+		},
 	}
 
 	for _, test := range tests {