Improve ErrorMassage on null-object in MapReducer

[SlowMo24]

Problem
A hard to decipher NullPointerException is thrown if the return-element of the map-step of a MapReducer is null and .uniq() is called.

Favorite solution
null-objects should simply be possible as a result, also for uniq().

Alternatives
The error-message should be improved and the javadoc of map as well as uniq should be updated, to inform users to not generate null-values.

[tyrasd]

do you have a few lines of code at hand that trigger the null pointer exception?

//edit: ah, got it. Anything like this triggers it:

        …
        .map(x -> null)
        .uniq();

//edit2: the null pointer exception is thrown here:

https://github.com/GIScience/oshdb/blob/51718d5/oshdb-api/src/main/java/org/heigit/bigspatialdata/oshdb/api/mapreducer/MapReducer.java#L1931

[tyrasd]

a slightly off topic, but related general advice: please try to simply avoid using the null construct (not only) in java. If you need a special "thing" to represent "missing data" (or something similar), consider the null object pattern instead.