[SPARK-59477][SQL] Add CSV write option treatNullAsEmptyString - #58767

### What changes were proposed in this pull request?

Adds a per-write CSV option `treatNullAsEmptyString` that overrides the
session-level `spark.sql.legacy.nullValueWrittenAsQuotedEmptyStringCsv` config
for a single write.

- When set to `false`, nulls are written as a bare, unquoted empty token while
  empty strings stay quoted (`""`), so downstream consumers can tell a null
  apart from an actual empty string.
- When set to `true`, nulls are written through the `emptyValue` (a quoted
  empty string `""` by default).
- When unset (including an explicit `null` value), the session config decides
  the behavior, so existing writes are unaffected.

The option is parsed via the existing `getBool` helper, so a non-boolean value
raises the structured `paramIsNotBooleanValueError` naming the option rather
than a bare error. It only takes effect when `nullValue` is left at its default
(empty string); a non-empty `nullValue` is written verbatim, making the option
a no-op, matching how the SQL config it overrides already composes with
`nullValue`.

### Why are the changes needed?

The behavior of writing a null vs. an empty string in CSV is currently only
controllable through the session-level
`spark.sql.legacy.nullValueWrittenAsQuotedEmptyStringCsv` config. A workload
that needs to differentiate null from an empty string in its output cannot do
so per-write without changing the session/workspace default, which is not
always settable. A per-write option lets a single write opt into the
differentiation without affecting other writes.

### Does this PR introduce _any_ user-facing change?

Yes, it adds a new CSV write option `treatNullAsEmptyString`. Behavior is
unchanged when the option is not set (the session config continues to decide),
so existing writes are unaffected.

### How was this patch tested?

Added unit tests in `CSVSuite` covering: the option winning over the session
config for both `true`/`false`, the no-op behavior when a non-empty `nullValue`
is set, explicit-null handling (falls back to the session config), and the
structured error raised for a non-boolean value. Also updated the
`validate CSV Options` test for the new option.

### Was this patch authored or co-authored using generative AI tooling?

Generated-by: Isaac

This pull request and its description were written by Isaac.

Closes #58767 from dejankrak-db/csv-treat-null-as-empty-string.

Authored-by: Dejan Krakovic <[email protected]>
Signed-off-by: Uros Bojanic <[email protected]>
(cherry picked from commit 66737e0)
Signed-off-by: Uros Bojanic <[email protected]>