Spring Remove incorrect @Nullable in DynamicPropertiesContextCustomizer

Hello~ This PR contains the following points： * Remove incorrect @Nullable annotation in buildDynamicPropertiesMap, this method may return empty map, but not null. * If the methods parameter of Constructor of DynamicPropertiesContextCustomizer is null, it will produce NPE, and if the parameter is a empty set, it doesn't seem to make much sense, considering that this class is visible by the package, so I suggest adding a empty check in this Constructor.

Comment From: chenqimiao

@sbrannen I have submitted the update, thank you for your pointing.

Comment From: sbrannen

This has been merged into master.

Thanks