Fix id based mapping for non-lowercase fields

[phd3]

Fixes the issue discussed in Slack thread. The change in #6520 causes nested column reads on ORC files to fail when they have non-lowercase field names.

io.trino.spi.TrinoException: Error opening Iceberg split s3a://REDACTED/data/timestamp_day=2021-01-21/00002-17-5a427ab9-81ab-442b-ad46-512af01a5019-00001.orc (offset=0, length=1479520): null
	at io.trino.plugin.iceberg.IcebergPageSourceProvider.createOrcPageSource(IcebergPageSourceProvider.java:354)
	at io.trino.plugin.iceberg.IcebergPageSourceProvider.createDataPageSource(IcebergPageSourceProvider.java:205)
	at io.trino.plugin.iceberg.IcebergPageSourceProvider.createPageSource(IcebergPageSourceProvider.java:167)
	at io.trino.spi.connector.ConnectorPageSourceProvider.createPageSource(ConnectorPageSourceProvider.java:68)
	at io.trino.plugin.base.classloader.ClassLoaderSafeConnectorPageSourceProvider.createPageSource(ClassLoaderSafeConnectorPageSourceProvider.java:66)
	at io.trino.split.PageSourceManager.createPageSource(PageSourceManager.java:64)
	at io.trino.operator.ScanFilterAndProjectOperator$SplitToPages.process(ScanFilterAndProjectOperator.java:254)
	at io.trino.operator.ScanFilterAndProjectOperator$SplitToPages.process(ScanFilterAndProjectOperator.java:182)
	at io.trino.operator.WorkProcessorUtils$3.process(WorkProcessorUtils.java:319)
	at io.trino.operator.WorkProcessorUtils$ProcessWorkProcessor.process(WorkProcessorUtils.java:372)
	at io.trino.operator.WorkProcessorUtils$3.process(WorkProcessorUtils.java:306)
	at io.trino.operator.WorkProcessorUtils$ProcessWorkProcessor.process(WorkProcessorUtils.java:372)
	at io.trino.operator.WorkProcessorUtils$3.process(WorkProcessorUtils.java:306)
	at io.trino.operator.WorkProcessorUtils$ProcessWorkProcessor.process(WorkProcessorUtils.java:372)
	at io.trino.operator.WorkProcessorUtils.getNextState(WorkProcessorUtils.java:221)
	at io.trino.operator.WorkProcessorUtils.lambda$processStateMonitor$2(WorkProcessorUtils.java:200)
	at io.trino.operator.WorkProcessorUtils$ProcessWorkProcessor.process(WorkProcessorUtils.java:372)
	at io.trino.operator.WorkProcessorUtils.getNextState(WorkProcessorUtils.java:221)
	at io.trino.operator.WorkProcessorUtils.lambda$finishWhen$3(WorkProcessorUtils.java:215)
	at io.trino.operator.WorkProcessorUtils$ProcessWorkProcessor.process(WorkProcessorUtils.java:372)
	at io.trino.operator.WorkProcessorSourceOperatorAdapter.getOutput(WorkProcessorSourceOperatorAdapter.java:149)
	at io.trino.operator.Driver.processInternal(Driver.java:387)
	at io.trino.operator.Driver.lambda$processFor$9(Driver.java:291)
	at io.trino.operator.Driver.tryWithLock(Driver.java:683)
	at io.trino.operator.Driver.processFor(Driver.java:284)
	at io.trino.execution.SqlTaskExecution$DriverSplitRunner.processFor(SqlTaskExecution.java:1075)
	at io.trino.execution.executor.PrioritizedSplitRunner.process(PrioritizedSplitRunner.java:163)
	at io.trino.execution.executor.TaskExecutor$TaskRunner.run(TaskExecutor.java:484)
	at io.trino.$gen.Trino_352_214_gef3079c____20210304_212845_2.run(Unknown Source)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630)
	at java.base/java.lang.Thread.run(Thread.java:832)
Caused by: java.lang.NullPointerException: undefined
	at io.trino.plugin.iceberg.IcebergPageSourceProvider$IdBasedFieldMapper.get(IcebergPageSourceProvider.java:418)
	at io.trino.orc.reader.StructColumnReader.<init>(StructColumnReader.java:104)
	at io.trino.orc.reader.ColumnReaders.createColumnReader(ColumnReaders.java:75)
	at io.trino.orc.OrcRecordReader.createColumnReaders(OrcRecordReader.java:579)
	at io.trino.orc.OrcRecordReader.<init>(OrcRecordReader.java:244)
	at io.trino.orc.OrcReader.createRecordReader(OrcReader.java:308)
	at io.trino.plugin.iceberg.IcebergPageSourceProvider.createOrcPageSource(IcebergPageSourceProvider.java:315)
	... 31 more

The added test here fails without this change with a similar stacktrace.

[findepi]

Can this be removed now?
if Spark Iceberg still writes incorrect file sizes, we should reconsider whether iceberg.use-file-size-from-metadata shouldn't be false by default?