`isNullOrBlank` can not be used with nullable strings

[JLLeitschuh]

The following test does not compile but should:

val message: String? = null
assertThat(message, isNullOrBlank)

[SirWellington]

I agree. I had to make my own Matcher:

val isNull: Matcher<Any?> = Matcher(Objects::isNull)

val isNullOrEmptyString: Matcher<CharSequence?> = isNull.or(isEmptyString as Matcher<CharSequence?>)

And use it as such:

assertThat(message, isNullOrEmptyString)
assertThat(message, !isNullOrEmptyString)

[npryce]

Fixed by #43