Fix to null types as schema instantiation argument

[DaniyelMe]

Creating a new instance of Schema with null argument should not be acceptable as a valid object.
This fix will make sure that new Schema(null); will throw an error "Schema must be an object".

Adding a test to cover this case.

[codecov]

Codecov Report

Merging #10 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #10   +/-   ##
=======================================
  Coverage   99.25%   99.25%           
=======================================
  Files           7        7           
  Lines         135      135           
  Branches       48       48           
=======================================
  Hits          134      134           
  Misses          1        1           

Impacted Files	Coverage Δ
src/index.js	99.11% <100.00%> (ø)

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