FOP warning causing a crash

<fo:table-cell width="" border-left="0.5pt green dashed">
<fo:block>Cell</fo:block>
</fo:table-cell>

FOP issues a warning because border-left has an invalid structure (the border-style should be placed before the border-color).

In FObj.warnOnUnknownProperties(), there is a call to getAttributeNameForValue() to find the name of the attribute whose value is unknown, then it stumbles on the width="" property, which returns a null object when instantiated. And there goes a NPE which leads to a crash:

if (prop.equals(value)) {
return attributeName;
}

I attached a patch which encloses the above code with a simple if-test.