[BUG] Extending a state with an empty body triggers StopIteration in state compiler

Description
An empty state declaration (e.g. empty_declaration: {}) is legal, and is a no-op. Extending the state (via extends) with a similarly empty body triggers a StopIteration in the state compiler.

Setup
(Please provide relevant configs and/or SLS files (be sure to remove sensitive info. There is no general set-up of Salt.)

Please be as specific as possible and give set-up details.

• on-prem machine
• VM (Virtualbox, KVM, etc. please specify)
• VM running on a cloud service, please be explicit and add details
• container (Kubernetes, Docker, containerd, etc. please specify)
• or a combination, please be explicit
• jails if it is FreeBSD
• classic packaging
• onedir packaging
• used bootstrap to install

Steps to Reproduce the behavior

extend_fail.sls:

empty_declaration: {}

extend:
  empty_declaration: {}

State application:

$ salt-call state.apply extend_fail saltenv=base
[ERROR   ] An un-handled exception was caught by Salt's global exception handler:
StopIteration: 
Traceback (most recent call last):
  File "/opt/salt/salt-call", line 23, in <module>
    sys.exit(salt_call())
  File "/opt/salt/lib/python3.10/site-packages/salt/scripts.py", line 443, in salt_call
    client.run()
  File "/opt/salt/lib/python3.10/site-packages/salt/cli/call.py", line 50, in run
    caller.run()
  File "/opt/salt/lib/python3.10/site-packages/salt/cli/caller.py", line 95, in run
    ret = self.call()
  File "/opt/salt/lib/python3.10/site-packages/salt/cli/caller.py", line 202, in call
    ret["return"] = self.minion.executors[fname](
  File "/opt/salt/lib/python3.10/site-packages/salt/loader/lazy.py", line 159, in __call__
    ret = self.loader.run(run_func, *args, **kwargs)
  File "/opt/salt/lib/python3.10/site-packages/salt/loader/lazy.py", line 1245, in run
    return self._last_context.run(self._run_as, _func_or_method, *args, **kwargs)
  File "/opt/salt/lib/python3.10/site-packages/salt/loader/lazy.py", line 1260, in _run_as
    return _func_or_method(*args, **kwargs)
  File "/opt/salt/lib/python3.10/site-packages/salt/executors/direct_call.py", line 10, in execute
    return func(*args, **kwargs)
  File "/opt/salt/lib/python3.10/site-packages/salt/loader/lazy.py", line 159, in __call__
    ret = self.loader.run(run_func, *args, **kwargs)
  File "/opt/salt/lib/python3.10/site-packages/salt/loader/lazy.py", line 1245, in run
    return self._last_context.run(self._run_as, _func_or_method, *args, **kwargs)
  File "/opt/salt/lib/python3.10/site-packages/salt/loader/lazy.py", line 1260, in _run_as
    return _func_or_method(*args, **kwargs)
  File "/opt/salt/lib/python3.10/site-packages/salt/modules/state.py", line 833, in apply_
    return sls(mods, **kwargs)
  File "/opt/salt/lib/python3.10/site-packages/salt/modules/state.py", line 1479, in sls
    ret = st_.state.call_high(high_, orchestration_jid)
  File "/opt/salt/lib/python3.10/site-packages/salt/state.py", line 3493, in call_high
    high, ext_errors = self.reconcile_extend(high)
  File "/opt/salt/lib/python3.10/site-packages/salt/state.py", line 1731, in reconcile_extend
    state_type = next(x for x in body if not x.startswith("__"))
StopIteration
Traceback (most recent call last):
  File "/opt/salt/salt-call", line 23, in <module>
    sys.exit(salt_call())
  File "/opt/salt/lib/python3.10/site-packages/salt/scripts.py", line 443, in salt_call
    client.run()
  File "/opt/salt/lib/python3.10/site-packages/salt/cli/call.py", line 50, in run
    caller.run()
  File "/opt/salt/lib/python3.10/site-packages/salt/cli/caller.py", line 95, in run
    ret = self.call()
  File "/opt/salt/lib/python3.10/site-packages/salt/cli/caller.py", line 202, in call
    ret["return"] = self.minion.executors[fname](
  File "/opt/salt/lib/python3.10/site-packages/salt/loader/lazy.py", line 159, in __call__
    ret = self.loader.run(run_func, *args, **kwargs)
  File "/opt/salt/lib/python3.10/site-packages/salt/loader/lazy.py", line 1245, in run
    return self._last_context.run(self._run_as, _func_or_method, *args, **kwargs)
  File "/opt/salt/lib/python3.10/site-packages/salt/loader/lazy.py", line 1260, in _run_as
    return _func_or_method(*args, **kwargs)
  File "/opt/salt/lib/python3.10/site-packages/salt/executors/direct_call.py", line 10, in execute
    return func(*args, **kwargs)
  File "/opt/salt/lib/python3.10/site-packages/salt/loader/lazy.py", line 159, in __call__
    ret = self.loader.run(run_func, *args, **kwargs)
  File "/opt/salt/lib/python3.10/site-packages/salt/loader/lazy.py", line 1245, in run
    return self._last_context.run(self._run_as, _func_or_method, *args, **kwargs)
  File "/opt/salt/lib/python3.10/site-packages/salt/loader/lazy.py", line 1260, in _run_as
    return _func_or_method(*args, **kwargs)
  File "/opt/salt/lib/python3.10/site-packages/salt/modules/state.py", line 833, in apply_
    return sls(mods, **kwargs)
  File "/opt/salt/lib/python3.10/site-packages/salt/modules/state.py", line 1479, in sls
    ret = st_.state.call_high(high_, orchestration_jid)
  File "/opt/salt/lib/python3.10/site-packages/salt/state.py", line 3493, in call_high
    high, ext_errors = self.reconcile_extend(high)
  File "/opt/salt/lib/python3.10/site-packages/salt/state.py", line 1731, in reconcile_extend
    state_type = next(x for x in body if not x.startswith("__"))
StopIteration

Expected behavior
The above example compiles cleanly, and (expectedly) does nothing, i.e.

local:

Summary for local
-----------
Succeeded: 0
Failed:   0
-----------
Total states run:    0
Total run time:  0.000 ms

Versions Report

Salt Version:
          Salt: 3006.3
 
Python Version:
        Python: 3.10.13 (main, Sep  6 2023, 02:16:00) [Clang 14.0.0 (clang-1400.0.29.202)]
 
Dependency Versions:
          cffi: 1.14.6
      cherrypy: 18.6.1
      dateutil: 2.8.0
     docker-py: Not Installed
         gitdb: 4.0.5
     gitpython: 3.1.32
        Jinja2: 3.1.2
       libgit2: Not Installed
  looseversion: 1.0.2
      M2Crypto: Not Installed
          Mako: Not Installed
       msgpack: 1.0.2
  msgpack-pure: Not Installed
  mysql-python: Not Installed
     packaging: 22.0
     pycparser: 2.21
      pycrypto: Not Installed
  pycryptodome: 3.9.8
        pygit2: Not Installed
  python-gnupg: 0.4.8
        PyYAML: 6.0.1
         PyZMQ: 23.2.0
        relenv: 0.13.10
         smmap: 3.0.2
       timelib: 0.2.4
       Tornado: 4.5.3
           ZMQ: 4.3.4
 
System Versions:
          dist: darwin 22.6.0 
        locale: utf-8
       machine: x86_64
       release: 22.6.0
        system: Darwin
       version: 13.5.1 x86_64

Additional context
This issue does not present in Salt 3002.x

git blame suggests that the regression was introduced in the following commit: https://github.com/saltstack/salt/commit/8ec0c32514587869cad565cf4d58e6027c828dfa

Start in salt/state.py at reconcile_extend, where the reported StopIteration occurs, and trace the call from salt/modules/state.py through state.apply. Reproduce the failure with the provided extend_fail.sls example and review commit 8ec0c32514587869cad565cf4d58e6027c828dfa. Done means the empty extension compiles cleanly and produces the expected zero-state summary.