Snapshot fails when encountering null MySQL TIME fields

It appears the new method SnapshotReader.readTimeField does not account for null values. It throws this exception and the snapshot fails.

```
2017-12-20 11:51:26,115] ERROR Failed due to error: Aborting snapshot due to error when last running 'COMMIT': null (io.debezium.connector.mysql.SnapshotReader:157)
org.apache.kafka.connect.errors.ConnectException
at io.debezium.connector.mysql.AbstractReader.wrap(AbstractReader.java:178)
at io.debezium.connector.mysql.AbstractReader.failed(AbstractReader.java:156)
at io.debezium.connector.mysql.SnapshotReader.execute(SnapshotReader.java:664)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NullPointerException
at io.debezium.connector.mysql.SnapshotReader.readTimeField(SnapshotReader.java:164)
at io.debezium.connector.mysql.SnapshotReader.readField(SnapshotReader.java:147)
at io.debezium.connector.mysql.SnapshotReader.lambda$execute$12(SnapshotReader.java:532)
at io.debezium.jdbc.JdbcConnection.query(JdbcConnection.java:389)
at io.debezium.connector.mysql.SnapshotReader.execute(SnapshotReader.java:521)
... 1 more
[2017-12-20 11:51:26,309] ERROR Task iready_snapper5_dbz-0 threw an uncaught and unrecoverable exception (org.apache.kafka.connect.runtime.WorkerTask:148)
org.apache.kafka.connect.errors.ConnectException
at io.debezium.connector.mysql.AbstractReader.wrap(AbstractReader.java:178)
at io.debezium.connector.mysql.AbstractReader.failed(AbstractReader.java:156)
at io.debezium.connector.mysql.SnapshotReader.execute(SnapshotReader.java:664)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NullPointerException
at io.debezium.connector.mysql.SnapshotReader.readTimeField(SnapshotReader.java:164)
at io.debezium.connector.mysql.SnapshotReader.readField(SnapshotReader.java:147)
at io.debezium.connector.mysql.SnapshotReader.lambda$execute$12(SnapshotReader.java:532)
at io.debezium.jdbc.JdbcConnection.query(JdbcConnection.java:389)
at io.debezium.connector.mysql.SnapshotReader.execute(SnapshotReader.java:521)
... 1 more
```