Wie kann ich die Ausgabe von Biblatex in Abhängigkeit von leeren Feldern einstellen?

% Filecontents; handle with care!
\begin{filecontents}{\jobname.bbx}
\ProvidesFile{\jobname.bbx}[2014/12/16 v0.1 TeXwelt test]
\RequireBibliographyStyle{numeric}

\DeclareNameAlias{mydate}{default}
\DefineBibliographyStrings{english}{version={Rev.}}

\renewcommand*{\newunitpunct}{\addsemicolon\addspace}

\DeclareFieldFormat[report]{title}{\mkbibquote{#1}}

\DeclareBibliographyDriver{report}{%
    \printlist{institution}%
    \newunit%
    \printnames{author}%
    \newunit%
    \printfield{title}%
    \newunit
    \printfield{number}%
    \newunit%
    \iffieldundef{version}{%
        {\color{red} sorry, no version available}%
    }{%
        \printfield{version}%
    }%
    \newunit%
    \iffieldundef{day}{}{%
        \printfield{day}.\addspace}%
        \printfield{month}
    \printfield{year}%
    \finentry%
}
\endinput
\end{filecontents}
\begin{filecontents}{\jobname.bib}
    @REPORT{APOSTO,
        author      = {G. Apostolakis},
        title       = {Data specialization for plant specific risk studies},
        institution = {Nuclear Engineering and Design 56},
        number      = {321-329},
        date        = {1979-07-12}
    }

    @REPORT{RiskSpectrumTheoryManual,
        author      = {Relcon Scandpower AB},
        title       = {RiskSpectrum Analysis Tools - Theory Manual},
        number      = {BET/07/0286},
        version     = {3.2.1},
        date        = {2012-08}
    }
\end{filecontents}
\documentclass{scrartcl}
\usepackage[bibstyle=\jobname,backend=biber]{biblatex}
\addbibresource{\jobname.bib}
\usepackage{xcolor}
\begin{document}
no version: \cite{APOSTO}\par
no day: \cite{RiskSpectrumTheoryManual}\par
\printbibliography
\end{document}