Fix empty container creation

[FrancoisWagner]

OCF writer lazily writes header on first write instead of file open changed the container writer behaviour to write the header lazily on first write. Consequently, it has prevented the possibility of creating valid empty containers although the documentation of the constructor still shows:

//  A schema string must be passed to ensure that a correct header is written even if no records are written. This
//  is required to produce valid empty Avro container files.

Prior fix:

avro getschema /tmp/testv10.avro
log4j:WARN No appenders could be found for logger (org.apache.hadoop.metrics2.lib.MutableMetricsFactory).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
Exception in thread "main" java.io.IOException: Not a data file.
        at org.apache.avro.file.DataFileStream.initialize(DataFileStream.java:102)
        at org.apache.avro.file.DataFileReader.<init>(DataFileReader.java:97)
        at org.apache.avro.tool.DataFileGetSchemaTool.run(DataFileGetSchemaTool.java:47)
        at org.apache.avro.tool.Main.run(Main.java:87)
        at org.apache.avro.tool.Main.main(Main.java:76)
Caused by: java.io.EOFException
        at org.apache.avro.io.BinaryDecoder$InputStreamByteSource.readRaw(BinaryDecoder.java:827)
        at org.apache.avro.io.BinaryDecoder.doReadBytes(BinaryDecoder.java:349)
        at org.apache.avro.io.BinaryDecoder.readFixed(BinaryDecoder.java:302)
        at org.apache.avro.io.Decoder.readFixed(Decoder.java:150)
        at org.apache.avro.file.DataFileStream.initialize(DataFileStream.java:100)
        ... 4 more

With fix:

avro getschema /tmp/testv10.avro
log4j:WARN No appenders could be found for logger (org.apache.hadoop.metrics2.lib.MutableMetricsFactory).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
{
  "type" : "record",
  "name" : "DummyRecord",
  "fields" : [ {
    "name" : "id",
    "type" : [ "null", "long" ],
    "default" : null
  }, {
    "name" : "name",
    "type" : [ "null", "string" ],
    "default" : null
  } ]
}

[FrancoisWagner]

@actgardner I couldn't get hold on the Writer's unit tests location. Let me know if you want me to add a unit test somewhere.

[actgardner]

This LGTM @FrancoisWagner, I'll take a look at adding tests in the future. Thanks!