[FLink]Fixed handle of null values and add some usage document

[ChenYunHey]

LakesoulConvert hold a hashMap which the key is mongoDB filed's name and the value is filed's schema,the hashMap will put all the fields in,if the field's value is null,the hashmap wiil set the filed's schema null .When build a Struct,if the bsonDocument dose not exist the hashmap hold's filed,then struct will supplement the filed.

another,I fixed the SyncDatabase.Users can control the input of optional parameters of flink jdbc.

[ChenYunHey]

my mongoDB document like this：
[
{ _id: 1, name: null },
{ _id: 2, name: 'Bob' },
{ _id: 3, name: null },
{ _id: 4, name: null },
{ _id: 12, name: null },
{ _id: 13, age: null },
{ _id: 14, age: 33 },
{ _id: 16, age: null },
{ _id: 20, name: null },
{ _id: 21, name: 'ss' },
{ _id: 22, name: 'ss' },
{ _id: 24, name: null }
]
the parquet file when write
+-------+------------+
| _id | rowKinds |
|-------+------------|
| 20 | insert |
+-------+------------+
+-------+-------+------------+
| _id | age | rowKinds |
|-------+-------+------------|
| 4 | | insert |
| 14 | 33 | insert |
| 16 | | insert |
+-------+-------+------------+

+-------+--------+-------+------------+
| _id | name | age | rowKinds |
|-------+--------+-------+------------|
| 1 | | | insert |
| 2 | Bob | | insert |
| 3 | | | insert |
| 12 | | | insert |
| 21 | ss | | insert |
| 22 | ss | | insert |
| 24 | | | insert |
+-------+--------+-------+------------+