Improved property tests with null examples that reveals bugs

[luisggpina]

Hi!

In an effort to improve the coverage of the property tests, I improved the existing property tests to test null handling and found some bugs:

• testInsertion adds a bunch of keys mapping to null that cause a NullPointerException when balancing the tree:

Caused by: java.lang.NullPointerException                                                                 
        at org.cellx.logish.IntMap$Node.balanceLeft(IntMap.java:614)                                                                                                                                                
        at org.cellx.logish.IntMap$Node.without(IntMap.java:686)                                                                                                                                                    
        at org.cellx.logish.IntMap$Node.with(IntMap.java:641)                                                                                                                                                       
        at org.cellx.logish.IntMap.withAll(IntMap.java:338)                                                                                                                                                         
        at org.cellx.logishTest.IntMapTests.testInsertion(IntMapTests.java:26)    

• testDeletion attempts to delete keys from an empty map, which also causes NullPointerException as the delete operation sometimes returns an empty map and other times returns null:

Caused by: java.lang.NullPointerException                                                                 
        at org.cellx.logishTest.IntMapTests.testDeletion(IntMapTests.java:48)   

More info on how to replicate these issues:

[ERROR] Failures:                                                                                         
[ERROR]   IntMapTests.testDeletion Unexpected error in property testDeletion with args [[15]] and seeds [6787329991909443937]                                                                                       
[ERROR]   IntMapTests.testInsertion Unexpected error in property testInsertion with args [[83]] and seeds [3090420391447367513]