:PowerAssert should require at least one argument
:PowerAssert should require at least one argument

`:PowerAssert` previously emitted below error on empty argument.

```
E116: Invalid arguments for function <SNR>183__assert(, '')
E15: Invalid expression: <SNR>183__assert(, '')
```

This is because `:PowerAssert` accepts no arguments.  However, at least
one argument is needed to represent an expression of Vim script.  So I
think `:PowerAssert` should require at least one argument.
It improves the error message as below:

```
E471: Argument required
```

syohex/vital-power-assert,haya14busa/vital-power-assert