Если я напишу:
null instanceof SomeClass
В таком случае нужно ли делать проверку на null перед использованием instanceof?
Может ли в таком случае возникнуть исключение NullPointerException?
нет, здесь проверка не нужна.
при использовании такой записи instanceof всегда будет возвращать false.
Об этом также написано в спецификации языка java:
The type of the RelationalExpression operand of the instanceof operator must be a reference type or the null type, or a compile-time error occurs.
It is a compile-time error if the ReferenceType mentioned after the instanceof operator does not denote a reference type that is reifiable (§4.7).
If a cast of the RelationalExpression to the ReferenceType would be rejected as a compile-time error (§15.16), then the instanceof relational expression likewise produces a compile-time error. In such a situation, the result of the instanceof expression could never be true.
At run time, the result of the instanceof operator is true if the value of the RelationalExpression is not null and the reference could be cast to the ReferenceType without raising a ClassCastException. Otherwise the result is false.