[Java] JsonPrinter does not handle empty groups

[Audrius-GR]

Similar to #799, seems if the group is empty, you end up with just a [\n

Dummy message that I think reproduces this:

  <sbe:message name="myMessage" id="6">
    <field name="myId" id="2" type="uint32"/>
    <group name="myGroupInfo" id="7" dimensionType="groupSizeEncoding">
      <field name="internalId" id="2" type="idtype"/>
      <field name="otherId" id="3" type="idType"/>
    </group>
    <data name="rawStringId" id="100" type="varDataEncoding"/>
  </sbe:message>

results in:

{
    "myId": 0,
    "myGroupInfo": [
    "rawStringId": ""
}

[Audrius-GR]

It seems that the [\n is created by onGroupHeader, and ],\n should be created by onEndGroup, but onEndGroup is only called if there are more than 0 elements in the group.

[Audrius-GR]

I think the easiest way around this is to handle terminating the array in onGroupHeader if numInGroup = 0