Inhibit default EMAIL in derived exporter

[Viktor Rosenfeld <listuser36@gmail.com>]

Hi,

commit 16f12e0 changed how the EMAIL option is configured in a derived
exporter that also uses this keyword.

In `contrib/ox-koma-letter.el', the koma-letter exporter configures the default
of the EMAIL option to the variable `org-koma-letter-email'. However, since
commit 16f12e0, if the option is missing, the default is set to
`user-mail-address' which is the default for the standard LaTeX exporter.

The current code causes the following bug in koma-letter exporter: If EMAIL is
missing, and `org-koma-letter-email' is `nil', then the exporter should not set
a email address in the exported LaTeX file. The rationale is that the email
address is configured in an external LCO file (and can be overwritten for an
individual letter, if desired). However, the new behavior causes the email to
be set explicitly, overwriting the value from the LCO file.

Consider the following example:

#+BEGIN_SRC org
#+LATEX_CLASS: my-letter
#+LCO: DefaultAddress
#+END_SRC

The expected LaTeX-code is:

#+BEGIN_SRC latex
\documentclass{scrlttr2}
\LoadLetterOption{DefaultAddress}    % <--- email is set here
% ...
\begin{document}
% ...
\end{document}
#+END_SRC

However, since commit 16f12e0, the generated code is:

#+BEGIN_SRC latex
\documentclass{scrlttr2}
\LoadLetterOption{DefaultAddress}    % <--- email is set here
\setkomavar{fromemail}{he-sk@client204-235.wlan.hu-berlin.de}  % <--- email is overwritten here
% ...
\begin{document}
% ...
\end{document}
#+END_SRC

Is this a bug in the LaTeX exporter or is this the intended behavior? Should I
maybe use another keyword instead of EMAIL? I think I used SENDER instead of
AUTHOR because of a similar conflict.

Cheers,
Viktor