\UseOneTimeHook changes undeclared hook to non-empty

[muzimuzhi]

Brief outline of the bug

When used on an undeclared hook, \UseOneTimeHook changes it to non-empty. That is, \HookIfEmptyTF would use the false branch. \UseHook doesn't have this problem.

The hook becomes non-empty because its \__hook_next <hook> is defined by \UseOneTimeHook. Is this expected behavior? If so my fix in #1515 is partially wrong.

Found when digging into #1513.

Minimal example showing the bug

\RequirePackage{latexbug}       % <--should be always the first line (see CONTRIBUTING)!
\documentclass{article}

\ExplSyntaxOn
\cs_new_protected:Npn \test_hook:Nn #1#2
  {
    \iow_term:e {
      [before]~ if~empty:~ \hook_if_empty:nTF {#2} T
        { F \iow_newline: [before]~ next~code:~\cs_meaning:c { __hook_next~#2 } }
    }
    \iow_term:n { [execute]~#1 {#2} }
    #1 {#2}
    \iow_term:e {
      [after]~ if~empty:~ \hook_if_empty:nTF {#2} T
        { F \iow_newline: [after]~ next~code:~\cs_meaning:c { __hook_next~#2 } }
      \iow_newline:
    }
  }

\iow_log:n { !~debug }
\test_hook:Nn \hook_use_once:n { undeclared~hook }
\test_hook:Nn \hook_use_once:n { package/amsmath/after } % issue #1513

\test_hook:Nn \hook_use:n { undeclared~hook2 }
\ExplSyntaxOff

\begin{document}
\end{document}

[before] if empty: T
[execute] \hook_use_once:n {undeclared hook}
[after] if empty: F
[after] next code: \long macro:#1#2#3#4#5#6#7#8#9->\__hook_use_once_clear:n {un
declared hook}

[before] if empty: T
[execute] \hook_use_once:n {package/amsmath/after}
[after] if empty: F
[after] next code: \long macro:->\__hook_use_once_clear:n {package/amsmath/afte
r}

[before] if empty: T
[execute] \hook_use:n {undeclared hook2}
[after] if empty: T

Log file (required) and possibly PDF file

latex2e-UseOneTimeHook-changes-undeclared-hook-to-non-empty.log