Incremental skewness and other higher moments when provided equal values do not yield expected results

## Checklist

> Please ensure the following tasks are completed before filing a bug report.

- [x] Read and understood the [Code of Conduct][code-of-conduct].
- [x] Searched for existing issues and pull requests.

## Description

> Description of the issue.

Encountered a potential bug when using the incremental accumulators for skewness and kurtosis. Notably, when providing all equal values, the accumulators return `NaN`. However, for uniformly distributed values, the skewness is `0`.

## Related Issues

> Does this issue have any related issues?

No related issues in this repository. However, Pandas seems to have encountered a similar issue: https://github.com/pandas-dev/pandas/issues/18044.

## Questions

> Any questions for reviewers?

No.

## Other

> Any other information relevant to this issue? This may include screenshots, references, stack traces, sample output, and/or implementation notes.

No.

#### Demo

> If relevant, provide a link to a live demo.

N/A

#### Reproduction

> What steps are required to reproduce the unexpected output?

In order to reproduce this bug, do the following:

```javascript
var incrskewness = require( '@stdlib/stats/incr/skewness' );

var acc = incrskewnesss();

for ( var i = 0; i < 100; i++ ) {
acc( 10.0 );
}
console.log( acc() );
// => NaN
```

#### Expected Results

> What are the expected results?

The following results are expected:

```text
0
```

#### Actual Results

> What are the actual results?

The following are the actual results:

```text
NaN
```

#### Environments

> What environments are affected (e.g., `Node v0.4.x`, `Chrome`, `IE 11`)? If Node.js, include the `npm` version, operating system, and any other potentially relevant platform information.

All environments are affected.

[code-of-conduct]: https://github.com/stdlib-js/stdlib/blob/develop/CODE_OF_CONDUCT.md

Start at the @stdlib/stats/incr/skewness entry point and run the supplied JavaScript reproduction with 100 equal values. Read the related incremental higher-moment implementation and its existing tests, then verify that equal inputs produce the expected zero result rather than NaN.

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