Support nullable for foreign keys

[chriskormaris]

https://github.com/aileftech/snap-admin/blob/297224dafe486cb45666c4a33907ea1f4c94bc2d/src/main/java/tech/ailef/snapadmin/external/SnapAdmin.java#L270

Please change above lines to this:

Column[] columnAnnotations = f.getAnnotationsByType(Column.class);
if (columnAnnotations.length != 0) {
	Column col = columnAnnotations[0];
	return col.nullable();
}

JoinColumn[] joinColumnAnnotations = f.getAnnotationsByType(JoinColumn.class);
if (joinColumnAnnotations.length != 0) {
	JoinColumn joinCol = joinColumnAnnotations[0];
	return joinCol.nullable();
}

return true;