[BugFix] Fix NPE workgroup executor (backport #54492)

[mergify]

Why I'm doing:

Early unlock causes the other thread to return an uninitialized workgroup object. Then causes the NPE.

the rax is 0x00 (workgroup->executors) is nullptr

   0x00000000043203d6 <+486>:   mov    0x58(%r12),%rax
   0x00000000043203db <+491>:   mov    0x48(%r12),%rsi
   0x00000000043203e0 <+496>:   lea    -0x60(%rbp),%rdi
   0x00000000043203e4 <+500>:   mov    0x1b0(%rax),%rax
=> 0x00000000043203eb <+507>:   mov    0x58(%rax),%rdx

(gdb) info registers 
rax            0x0                 0
rbx            0x7f7a793f07c0      140163996911552
rcx            0x17                23
rdx            0x0                 0
rsi            0x7f7a7942a290      140163997147792
rdi            0x7f7955dcb2c0      140159108297408
rbp            0x7f7955dcb320      0x7f7955dcb320
rsp            0x7f7955dcb260      0x7f7955dcb260
r8             0x3                 3
r9             0xc2a               3114
r10            0x18                24

(gdb) p this
$34 = (starrocks::pipeline::FragmentExecutor * const) 0x7f7955dcb340
(gdb) p &this->_query_ctx
$37 = (starrocks::pipeline::QueryContext **) 0x7f7955dcb380
(gdb) p &this->_fragment_ctx
$38 = (starrocks::pipeline::FragmentContextPtr *) 0x7f7955dcb388
(gdb) p this
(gdb) p &this->_wg
$45 = (starrocks::workgroup::WorkGroupPtr *) 0x7f7955dcb398
(gdb) p (0x7f7955dcb340 - 0x7f7955dcb398)
$46 = -88
(gdb) p &this->_wg
$47 = (starrocks::workgroup::WorkGroupPtr *) 0x7f7955dcb398
(gdb) p 0x1b0
$54 = 432
(gdb) p &this->_wg._M_ptr[0]
$55 = (std::__shared_ptr<starrocks::workgroup::WorkGroup, (__gnu_cxx::_Lock_policy)2>::element_type *) 0x7f7a79329810
(gdb) p &this->_wg._M_ptr->_executors
$56 = (starrocks::workgroup::PipelineExecutorSet **) 0x7f7a793299c0
(gdb) p (0x7f7a79329810 - 0x7f7a793299c0)
$57 = -432

What I'm doing:

Fixes https://github.com/StarRocks/StarRocksTest/issues/9004

What type of PR is this:

• 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.
• 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:

• 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