fromFile not exist cause queue NPE

[WesleyOne]

plz close this issue if same as #90

org.apache.rocketmq.streams.examples.source.FileSourceExample

if fromFile is not exist, will throwing NPE as below.

java.lang.NullPointerException
	at org.apache.rocketmq.streams.common.channel.impl.file.FileSource.startSource(FileSource.java:90)
	at org.apache.rocketmq.streams.common.channel.source.AbstractSource.start(AbstractSource.java:114)
	at org.apache.rocketmq.streams.common.topology.ChainPipeline.startChannel(ChainPipeline.java:150)
	at org.apache.rocketmq.streams.client.transform.DataStream$7.run(DataStream.java:714)
	at java.lang.Thread.run(Thread.java:748)

how about fast-fail with throwing FileNotFoundException

[Kvicii]

@dongeforever hi there,
I think this kind of work to initialize capacity, if necessary, should be added with mandatory verification, but if the performance gap is not big, capacity initialization can be omitted

[dongeforever]

@Kvicii The countdownlatch capacity is not for performance.
BTW, verification is needed

[dongeforever]

@WesleyOne LGTM.
It's better to throw an exception when the initialization encounters an unexpected error.
It's not a good idea to treat the 'file not found error' as an empty stream.
you may submit a pr to fix it.