Substring Search and Unnest String throw a NullPointerException on an empty cell #7548

What happened?

Substring Search and Unnest String throw a NullPointerException when the column they are configured on is empty in any row. Both read the field and call toString on it with no null check: SubstringSearchOpExec.scala:33 is tuple.getField(desc.attribute).toString, and UnnestStringOpExec.scala:33 does the same inside its split call.

An empty value is ordinary input here. A blank CSV cell arrives as null, since univocity returns null for an empty field and AttributeTypeUtils.parseField passes it through by design, its first line being if (field == null) return null.

The execution then pauses on that row, with the exception shown in the operator's console.

Expected: a row with an empty value either passes through, or is skipped, or fails with a message naming the column. A NullPointerException is not one of those.

How to reproduce?

Upload a CSV with a blank cell:

id,text
1,hello world
2,
3,goodbye world

Build CSV File Scan to Substring Search, point Substring Search at text with substring world, and run. The execution pauses on row 2 and the operator's console shows the NullPointerException. Repeat with Unnest String on the same column.

Version/Branch

1.3.0-incubating-SNAPSHOT (main)

Relevant log output

[WARN] [WF35-SubstringSearch-c996db-main-0] [DataProcessor] [DP-thread] - Cannot invoke "Object.toString()" because the return value of "org.apache.texera.amber.core.tuple.Tuple.getField(String)" is null
org.apache.texera.amber.operator.substringSearch.SubstringSearchOpExec.findSubstring(SubstringSearchOpExec.scala:33)
org.apache.texera.amber.operator.filter.FilterOpExec.processTuple(FilterOpExec.scala:33)
org.apache.texera.amber.engine.architecture.worker.DataProcessor.processInputTuple(DataProcessor.scala:111)
org.apache.texera.amber.engine.architecture.worker.DPThread.$anonfun$runDPThreadMainLogic$2(DPThread.scala:211)