[common] Fix NullPointerExceptions on null transform inputs

[plusplusjiajia]

Purpose

Three NullPointerExceptions on null transform inputs, all reachable today:

• TrimTransform reads charsToTrim with toString() before checking it for null, so a null second input throws — even though StringUtils.ltrim/rtrim, which it delegates to, already define a null charsToTrim as a null result.
• SubstringTransform reads a begin or length field with InternalRow.getInt without an isNullAt check. On a GenericRow that throws; on a columnar row it returns an undefined value, so the rule masks at an arbitrary offset without failing.
• StringTransform.toString maps inputs with Object::toString and throws on a null input, which ConcatTransform and ConcatWsTransform accept.

All three change behaviour only for inputs that previously threw, so no query that previously succeeded changes its result, and nothing about the persisted JSON format changes.