Latex3: How to test if macro is empty?

Question

How to test if a macro is empty? As you can see in MWE, the creator1email is empty (or \relax ?), but my test via \cs_if_exist get an 1.

\documentclass{article}

\ExplSyntaxOn
    \int_new:N \l_creator_int
    \NewDocumentCommand\creator{ O{} m}
    {
        \int_incr:N \l_creator_int
        \cs_new_protected:cpn {creator\int_use:N\l_creator_int}{#2}
        \cs_new_protected:cpn {creator\int_use:N\l_creator_int email}{#1}
    }
    
\ExplSyntaxOff


\begin{document}


\creator[]{Me}

\ExplSyntaxOn
    \cs_if_exist:cTF {creator1email}  {1} {0}

     \use:c {creator1email}
\ExplSyntaxOff

\end{document}

Answer 1

Please note that expl3 offers you different data types and also provides tests for those data types. One of these is a token list, which is the correct variable type to store contents which you might want to print out.

The following changes your code to follow expl3 naming conventions and uses token list variables to store the email addresses. As a result you can use \tl_if_empty:cTF to check whether the email is indeed empty.

\documentclass{article}

\ExplSyntaxOn
\int_new:N \l_sergiokapone_creator_int
\cs_new_protected:Npn \sergiokapone_new_creator:nn #1#2
  {
    \int_incr:N \l_sergiokapone_creator_int
    \tl_new:c
      { l__sergiokapone_creator \int_use:N \l_sergiokapone_creator_int _tl }
    \tl_new:c
      { l__sergiokapone_email \int_use:N \l_sergiokapone_creator_int _tl }
    \tl_set:cn
      { l__sergiokapone_creator \int_use:N \l_sergiokapone_creator_int _tl }
      {#2}
    \tl_set:cn
      { l__sergiokapone_email \int_use:N \l_sergiokapone_creator_int _tl }
      {#1}
  }
\NewDocumentCommand \creator { O{} m }
  { \sergiokapone_new_creator:nn {#1} {#2} }

\NewDocumentCommand\usemail { m }
  {
    \tl_if_empty:cTF { l__sergiokapone_email #1 _tl }
      { \emph{empty} }
      { \tl_use:c { l__sergiokapone_email #1 _tl } }
  }

\ExplSyntaxOff


\begin{document}


\creator[]{Me}
\creator{You}
\creator[[email protected]]{Him}

\usemail{1}
\usemail{2}
\usemail{3}

\end{document}