Fix zeroth fibonacci and negative number handling

fib.py

@@ -1,15 +1,19 @@
-
 """
 Fibonacci number generator
 When given a position, the function returns the fibonacci at that position in the sequence.
 The zeroth number in the fibonacci sequence is 0. The first number is 1
 Negative numbers should return None
 """
 def fibonacci(position):
-  if(position == 1 or position == 2):
+  # Handle negative numbers
+  if position < 0:
+    return None
+
+  # Handle base cases
+  if position == 0:
+    return 0
+  if position == 1:
     return 1
-  return fibonacci(position - 1) + fibonacci(position - 2)
-
-
-
-
+
+  # Recursive case
+  return fibonacci(position - 1) + fibonacci(position - 2)