source-mysql: Add tests of zero-valued date/time columns

Apparently these are special-cased in the binary value encoding
and up until a couple months ago there was a bug in how times
were handled, so it's worth explicitly verifying these.

source-mysql/datatype_test.go

@@ -89,7 +89,9 @@ func TestDatatypes(t *testing.T) {
 		{ColumnType: "set('a', 'b', 'c')", ExpectType: `{"type":["string","null"]}`, InputValue: "a,c", ExpectValue: `"a,c"`},
 
 		{ColumnType: "date", ExpectType: `{"type":["string","null"]}`, InputValue: "1991-08-31", ExpectValue: `"1991-08-31"`},
+		{ColumnType: "date", ExpectType: `{"type":["string","null"]}`, InputValue: "0000-00-00", ExpectValue: `"0000-00-00"`},
 		{ColumnType: "time", ExpectType: `{"type":["string","null"]}`, InputValue: "765:43:21", ExpectValue: `"765:43:21"`},
+		{ColumnType: "time", ExpectType: `{"type":["string","null"]}`, InputValue: "00:00:00", ExpectValue: `"00:00:00"`},
 		{ColumnType: "year", ExpectType: `{"type":["integer","null"]}`, InputValue: "2003", ExpectValue: `2003`},
 
 		// The DATETIME column type will be stored verbatim by MySQL, and we will interpret it as being in