fix bug introduced by #603, because an empty field is now a string
fix bug introduced by #603, because an empty field is now a string

ilastik/conda-build,ilastik/conda-build,mwcraig/conda-build,sandhujasmine/conda-build,rmcgibbo/conda-build,sandhujasmine/conda-build,ilastik/conda-build,mwcraig/conda-build,sandhujasmine/conda-build,rmcgibbo/conda-build,mwcraig/conda-build,dan-blanchard/conda-build,rmcgibbo/conda-build,dan-blanchard/conda-build,dan-blanchard/conda-build