Handle nil values similar to how Clojure does for #63

[vincentjames501]

This potentially addresses #63. The goal is to make the behavior on nil the exact same as nil would be handled with Clojure equivalent functions.

The above behavior works like the following:

CSK	CLJ	Nil input behavior
-><case>-keyword	keyword	nil
-><case>-string	str	""
-><case>-symbol	symbol	IllegalArgumentException
-><case>	identity	nil

I could also see the argument for simply returning nil for all of the above instead or if you truly feel nil is just a bad input, throwing IllegalArgumentException/NullPointerException would be fine and still better than an Error. Let me know if you prefer one more than the other and I'm happy to adjust the PR.

(deftest nils-test
  (testing "returns idiomatic response for type converting functions on nil input"
    (is (nil? (csk/->kebab-case-keyword nil)))
    (is (= "" (csk/->kebab-case-string nil)))
    (is (thrown? IllegalArgumentException (csk/->kebab-case-symbol nil))))
  (testing "returns idiomatic response for non-type converting functions on nil input"
    (is (nil? (csk/->kebab-case nil)))))

For reference:

keyword

user=> (keyword nil)
nil
user=> (keyword "foo")
:foo
user=> (keyword :foo)
:foo
user=> (keyword 'foo)
:foo

symbol

user=> (symbol nil)
Execution error (IllegalArgumentException) at user/eval2101 (REPL:1).
no conversion to symbol

user=> (symbol "foo")
foo
user=> (symbol :foo)
foo
user=> (symbol 'foo)
foo

str

user=> (str nil)
""
user=> (str "foo")
"foo"
user=> (str :foo)
":foo"
user=> (str 'foo)
"foo"

name

user=> (name nil)
Execution error (NullPointerException) at user/eval2091 (REPL:1).
null

user=> (name "foo")
"foo"
user=> (name :foo)
"foo"
user=> (name 'foo)
"foo"

[qerub]

After much deliberation and opinion collection from #clj-commons, I decided that I think the least bad way to handle nil inputs is to throw an Exception (instead of an Error). I've changed the library to do so with #70 so I'm closing this PR. But I still think there's room for discussion so I'm leaving #63 open.