reject null input in longestCommonSubstringLengthArray

[dxbjavid]

LongestCommonSubsequence.longestCommonSubstringLengthArray dereferences left and right straight away to size the DP array, so passing a null sequence throws a raw NullPointerException out of the public method. Its siblings apply, logestCommonSubsequence and longestCommonSubsequence all guard null and throw IllegalArgumentException, and that behaviour is part of their documented contract, so a caller that catches IllegalArgumentException as the class leads it to expect is instead surprised by an NPE from this one method. Add the same null check and document it so the method lines up with the rest of the class.

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