ORC-2177: Fix array conversion with empty first batch

[cxzl25]

What changes were proposed in this pull request?

Add a batchSize > 0 guard in ConvertTreeReader.convertVector() before accessing index 0.

Why are the changes needed?

When reading an ORC file with schema evolution converting array<int> to array<string>, an ArrayIndexOutOfBoundsException is thrown if the first batch processed by the element reader has childCount=0 (i.e., all rows in the batch contain empty arrays).

java.lang.ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0
	at org.apache.orc.impl.ConvertTreeReaderFactory$StringGroupFromAnyIntegerTreeReader.setConvertVectorElement(ConvertTreeReaderFactory.java:1094)
	at org.apache.orc.impl.ConvertTreeReaderFactory$ConvertTreeReader.convertVector(ConvertTreeReaderFactory.java:305)
	at org.apache.orc.impl.ConvertTreeReaderFactory$StringGroupFromAnyIntegerTreeReader.nextVector(ConvertTreeReaderFactory.java:1115)
	at org.apache.orc.impl.TreeReaderFactory$ListTreeReader.nextVector(TreeReaderFactory.java:2892)
	at org.apache.orc.impl.reader.tree.StructBatchReader.readBatchColumn(StructBatchReader.java:66)
	at org.apache.orc.impl.reader.tree.StructBatchReader.nextBatchForLevel(StructBatchReader.java:101)
	at org.apache.orc.impl.reader.tree.StructBatchReader.nextBatch(StructBatchReader.java:78)
	at org.apache.orc.impl.RecordReaderImpl.nextBatch(RecordReaderImpl.java:1444)

How was this patch tested?

Added testIntArrayToStringArrayFirstBatchAllEmpty in TestConvertTreeReaderFactory

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