Fixes test_nil_is_an_object

* Filled-in blank to be 'true'
* This means that unlike NULL in other languages,
  nil is an object

[#65851396]
Fixes test_you_don't_get_null_pointer_errors_when
_calling_methods_on_nil

* Filled in 'NoMethodError' on line 16 because it
  was in the "answers you seek"
* Followed the hint and placed "undefined method"
  in the blank because it was expected to match
  "undefined method" in subsequent tests

[#65851396]
Fixes the first nil method on line 25

* Assumed nil.nil? is asking the nil object if it
  is nil. An equal test should be nil==nil

[#65851396]
Fixes the second nil test on line 26

* I tried "nil" first, but the exception thrown
  then said that "nil" was expected but was "".
  Realized that "" may be equal to nil as a
  string.

[#65851396]
Fixes the third nil test on line 27

* The exception hinted that the blank was expected
  but was "nil," so the blank also had to be "nil"

[#65851396]