[BugFix] split_part fuction should not return null when delimiter is not matched

[Mesut-Doner]

Why I'm doing:

What I'm doing:

Fixes #46774

What type of PR is this:

• [X] BugFix
• [ ] Feature
• [ ] Enhancement
• [ ] Refactor
• [ ] UT
• [ ] Doc
• [ ] Tool

Does this PR entail a change in behavior?

• [ ] Yes, this PR will result in a change in behavior.
• [x] No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• [ ] Interface/UI changes: syntax, type conversion, expression evaluation, display information
• [ ] Parameter changes: default values, similar parameters but with different default values
• [ ] Policy changes: use new policy to replace old one, functionality automatically enabled
• [ ] Feature removed
• [ ] Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• [x] I have added test cases for my bug fix or my new feature
• [ ] This pr needs user documentation (for new or modified features or behaviors)
  • [ ] I have added documentation for my new feature or new function
• [ ] This is a backport pr

Bugfix cherry-pick branch check:

• [ ] I have checked the version labels which the pr will be auto-backported to the target branch
  • [ ] 3.3
  • [ ] 3.2
  • [ ] 3.1
  • [ ] 3.0
  • [ ] 2.5

[ZiheLiu]

Hi @Mesut-Doner, the Clang-Format CI Test failed. Need to reformat it.

-#include "gtest/gtest.h"
+#include <memory>
+
 #include "column/binary_column.h"
 #include "column/column_helper.h"
 #include "exprs/string_functions.h"
-#include <memory>
+#include "gtest/gtest.h"
 
 namespace starrocks {
-class SplitPartTest : public ::testing::Test {
-};
+class SplitPartTest : public ::testing::Test {};
 
-void split_part_test(const std::string& haystack, const std::string& delimiter, int part_number, const std::string& expected) {
+void split_part_test(const std::string& haystack, const std::string& delimiter, int part_number,
+                     const std::string& expected) {
     auto context = std::make_unique<FunctionContext>();

[github-actions[bot]]

[FE Incremental Coverage Report]

:white_check_mark: pass : 0 / 0 (0%)

[github-actions[bot]]

[BE Incremental Coverage Report]

:white_check_mark: pass : 8 / 9 (88.89%)

file detail

	path	covered_line	new_line	coverage	not_covered_line_detail
:large_blue_circle:	be/src/exprs/split_part.cpp	8	9	88.89%	[145]

[Mesut-Doner]

Hello @ZiheLiu CI PIPELINE / ADMIT TEST (pull_request) is failing. How can I solve it?