Fix unmarshal null values for non-pointer fields

[neal]

#407 fixed the issue where an explicit null field did not reset the value, but a regression was introduced where previously it skipped any null values for non-pointer fields, that now returns an error.

This PR adds a IsNull() check before each unmarshal and the result is the same behavior as I see with stdlib now.

eg. stdlib ignores any null values for non-pointer fields: https://goplay.tools/snippet/dEJi7819RKc

The updated unit test also validates this behavior.

Without this change, the updated test fails to unmarshal:

--- FAIL: TestUnmarshalNull (0.00s)
    basic_test.go:341: easyjson.Unmarshal() error: parse error: expected string near offset 14 of 'String'
    basic_test.go:345: Wanted "bla", got ""
    basic_test.go:349: Wanted nil, got "bla"