Aggregation command earliest and latest throw NullPointerException if no data

[eemhu]

Describe the bug

No data generates NullPointerException.

org.apache.spark.sql.streaming.StreamingQueryException: Job aborted due to stage failure: Task 0 in stage 9.0 failed 4 times, most recent failure: Lost task 0.3 in stage 9.0 (TID 13, xxx, executor 21): java.lang.NullPointerException
    at com.teragrep.pth10.ast.commands.aggregate.UDAFs.BufferClasses.TimestampMapBuffer.earliest(TimestampMapBuffer.java:131)
    at com.teragrep.pth10.ast.commands.aggregate.UDAFs.EarliestLatestAggregator_String.finish(EarliestLatestAggregator_String.java:92)
    at com.teragrep.pth10.ast.commands.aggregate.UDAFs.EarliestLatestAggregator_String.finish(EarliestLatestAggregator_String.java:58)

Expected behavior

Should return no data, but also no exception.

How to reproduce

Try running earliest or latest to no rows.

Screenshots

Software version

4.17.0

Desktop (please complete the following information if relevant):

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Additional context

[eemhu]

internal pr created

[51-code]

Internal pr reviewed

[eemhu]

internal pr merged