Fix ReferenceField when used inside form

[fzaninotto]

Closes #4124

The bug is a bit tricky: FormInput decorates Field components with Labeled, passing them their props. Then, it renders the Field, passing them the record.

export const FormInput = ({ input, classes: classesOverride, ...rest }) => {
    const classes = useStyles({ classes: classesOverride });
    return input ? (
        <div
            className={classnames(
                'ra-input',
                `ra-input-${input.props.source}`,
                input.props.formClassName
            )}
        >
            {input.props.addLabel ? (
                <Labeled
                    id={input.props.id || input.props.source}
                    {...input.props}
                    {...sanitizeRestProps(rest)}
                >
                    {React.cloneElement(input, {
                        className: classnames(
                            {
                                [classes.input]: !input.props.fullWidth,
                            },
                            input.props.className
                        ),
                        id: input.props.id || input.props.source,
                        ...rest,
                    })}
                </Labeled>
            ) : (
                React.cloneElement(input, {
                    className: classnames(
                        {
                            [classes.input]: !input.props.fullWidth,
                        },
                        input.props.className
                    ),
                    id: input.props.id || input.props.source,
                    ...rest,
                })
            )}
        </div>
    ) : null;
};

The problem is that the record value for ReferenceField is {} due to defaultProps, so it overrides the record passed by FormInput.

Then you might ask: how come this used to work in 2.x? Well, my friend, that's because in 2.x we didn't export ReferenceField, but withStyles(styles)(ReferenceField), so the defaultProps of ReferenceField were never passed to the Labeled element...

And you might also ask: Why did we need a defaultValue in the first place? It probably dates before we used lodash.get to get the id form the record, or before the useGetMany implementation ignored undefined ids. Anyway, it's no longer necessary in 3.x.