Fix prefer-class-properties to ignore constructor overload declarations

[matwilko]

Description

When used in a Typescript codebase, where constructor overloads/multiple signatures are definable, prefer-class-properties will throw an exception, as it currently assumes that there will only be one or zero constructor declarations in a class (which is true for JS, but not for TS).

So the following results in an exception when executing ESLint

class A {

    constructor();
    constructor(a: number = 0) {
        // ...
    }
}

because getConstructor() will find the empty constructor overload/signature first, and then subsequently fail when trying to access the body, which is null.

I've updated getConstructor() to ignore constructors with no body, so that the "real" constructor is found and analysed.