skip null values in flatten

When flattening a list of collections, we can always skip null values,
even if the type of that value was unknown.

cty/function/stdlib/collection.go

@@ -536,6 +536,12 @@ func flattener(flattenList cty.Value) ([]cty.Value, []cty.ValueMarks, bool) {
 	for it := flattenList.ElementIterator(); it.Next(); {
 		_, val := it.Element()
 
+		if val.IsNull() {
+			// If the collection is null, there is nothing to append even if
+			// the type was unknown.
+			continue
+		}
+
 		// Any dynamic types could result in more collections that need to be
 		// flattened, so the type cannot be known.
 		if val.Type().Equals(cty.DynamicPseudoType) {

cty/function/stdlib/collection_test.go

@@ -2031,6 +2031,24 @@ func TestFlatten(t *testing.T) {
 			}),
 			"",
 		},
+		{
+			cty.TupleVal([]cty.Value{
+				cty.TupleVal([]cty.Value{
+					cty.StringVal("a"),
+					cty.StringVal("b"),
+				}),
+				cty.NullVal(cty.DynamicPseudoType),
+				cty.TupleVal([]cty.Value{
+					cty.StringVal("c"),
+				}),
+			}),
+			cty.TupleVal([]cty.Value{
+				cty.StringVal("a"),
+				cty.StringVal("b"),
+				cty.StringVal("c"),
+			}),
+			"",
+		},
 	}
 
 	for _, test := range tests {