As it is stated on your site (http://xml.apache.org/fop/fonts.html) embed-file 
attribute is optional but when I removed this attribute I catched an exception.
The method org.apache.fop.configuration.FontInfo.getEmbedFile() throws 
exception because if there is no attribute "embed-file" in tag "font" in 
config.xml then variable "private String embedFile" is null and there is no 
need of further actions.
To solve the problem you need change
    public String getEmbedFile() throws FOPException
    {
        try{
            embedFile = (new URL(embedFile)).getFile();
        }
        catch(MalformedURLException _ex) { }
        if((new File(embedFile)).isAbsolute())
            return embedFile;
        else
            return getBaseDir() + embedFile;
    }
to 
    public String getEmbedFile() throws FOPException
    {
	if(embedFile == null) return null;
        try{
            embedFile = (new URL(embedFile)).getFile();
        }
        catch(MalformedURLException _ex) { }
        if((new File(embedFile)).isAbsolute())
            return embedFile;
        else
            return getBaseDir() + embedFile;
    }

So I added line "if(embedFile == null) return null;" in the beginning of method 
getEmbedFile() in class org.apache.fop.configuration.FontInfo and solved this 
problem.

Yours sincerely,
Sergei Nemchaninov