Fix checkFormat fail for Null input
Fix checkFormat fail for Null input

When optional input is null we don't need to call
function checkFormat for this input.

SciDAP/cwltool,dleehr/cwltool,SciDAP/cwltool,common-workflow-language/cwltool,dleehr/cwltool,dleehr/cwltool,dleehr/cwltool,SciDAP/cwltool,common-workflow-language/cwltool,common-workflow-language/cwltool,SciDAP/cwltool