Provide some conditional helpers

[nesk]

We could provide some conditional helpers :

• ifNotNull
• ifInstanceOf

Instead of:

if (prop.unwrap() != null) prop { // this: Validatable<SomeClass?>
    constrain { it?.someBool == true }
}

we could write:

prop.ifNotNull { // this: Validatable<SomeClass>
    constrain { it.someBool }
}

We avoid calling unwrap, the receiver is automatically defined, and it's no longer nullable.

Some helpers unrelated to type casting could be useful too:

• ifNotEmpty (CharSequence and iterables)
• ifNotBlank
• ifNotNullOrEmpty
• ifNotNullOrBlank