feat: Add empty_as_null and keep_nulls flags to Expr.explode

[coastalwhite]

xref: #14423.

[codecov]

Codecov Report

❌ Patch coverage is 85.83815% with 49 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.05%. Comparing base (7d5bb7a) to head (b85d7ab).
⚠️ Report is 29 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-plan/src/dsl/format.rs	0.00%	12 Missing ⚠️
crates/polars-plan/src/plans/ir/tree_format.rs	0.00%	8 Missing ⚠️
crates/polars-core/src/chunked_array/mod.rs	68.75%	5 Missing ⚠️
crates/polars-core/src/series/ops/reshape.rs	60.00%	4 Missing ⚠️
crates/polars-expr/src/expressions/ternary.rs	0.00%	4 Missing ⚠️
.../polars-ops/src/chunked_array/strings/find_many.rs	80.00%	4 Missing ⚠️
crates/polars-ops/src/series/ops/is_in.rs	90.00%	4 Missing ⚠️
crates/polars-plan/src/dsl/expr/mod.rs	0.00%	2 Missing ⚠️
crates/polars-plan/src/plans/aexpr/builder.rs	0.00%	2 Missing ⚠️
crates/polars-expr/src/expressions/sortby.rs	87.50%	1 Missing ⚠️
... and 3 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25289      +/-   ##
==========================================
+ Coverage   81.82%   82.05%   +0.23%     
==========================================
  Files        1709     1711       +2     
  Lines      236481   237142     +661     
  Branches     3025     3011      -14     
==========================================
+ Hits       193501   194596    +1095     
+ Misses      42205    41773     -432     
+ Partials      775      773       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[orlp]

Just one nit.