[njoubert-cleverage]

Description

Fixes #365.

• ArrayElementTransformer (array_element): null when the index is outside the array, at either end. After the end, there is no more "Undefined array key" PHP warning (an exception in debug mode, null in production); before the start (negative index), the first element is no longer returned (what array_slice() gives).
• ArrayLastTransformer (array_last): null for an empty array, without warning.
• InputCsvReaderTask (input_csv_reader): an input that is not a path (e.g. null) throws an explicit UnexpectedValueException ("The input of the input_csv_reader task must be the path of the file to read, "null" given."), instead of an "Undefined array key "file_path"" PHP warning followed by a TypeError (dirname()).
• Documentation: rate n/a items/s of advanced_stat_counter, headers: [] of csv_reader / input_csv_reader (UnexpectedValueException), optional $accessor of ConditionTrait, ParsedExpression accepted by the condition of rules.
• Tests: ArrayElementTransformerTest, ArrayLastTransformerTest, CsvReaderTaskTest (they fail with the PHP warnings without the fix, failOnWarning).

Requirements

• Documentation updates
  • Reference
  • Changelog
• Unit tests

Breaking changes

Behaviour change, to be mentioned in the UPGRADE.md section of the release: in debug mode, array_element / array_last outside the array return null instead of failing; a negative index of array_element before the start of the array returns null instead of the first element.

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