TypeError: Path must be a string. Received null

### Description
there's an error at each execution of the command line

```
HTML SUMMARY
✖ 7 problem(s) found

Adding 33 link(s) to the queue...

▶ http://localhost:8000/prices/single-product.html

BASE single-product.html
STATUS 404 Not Found
ELAPSED 0.01 secs

❯ Checking HTML guidelines...

HTML SUMMARY
✔ no problem found

path.js:7
throw new TypeError('Path must be a string. Received ' + inspect(path));
^

TypeError: Path must be a string. Received null
at assertPath (path.js:7:11)
at Object.basename (path.js:1355:5)
at runValidator (/usr/local/lib/node_modules/w3c-link-validator/src/w3clink.js:70:22)
at Request._callback (/usr/local/lib/node_modules/w3c-link-validator/src/w3clink.js:117:13)
at Request.self.callback (/usr/local/lib/node_modules/w3c-link-validator/node_modules/request/request.js:185:22)
at emitTwo (events.js:106:13)
at Request.emit (events.js:191:7)
at Request. (/usr/local/lib/node_modules/w3c-link-validator/node_modules/request/request.js:1157:10)
at emitOne (events.js:96:13)
at Request.emit (events.js:188:7)
```

### Steps to Reproduce
1. `npm install w3c-link-validator -g`
2. `w3clink check http://localhost:8000`

**Expected behavior**: no error

**Actual behavior**: error

**Reproduces how often**: every single time

### Versions

**Package**: 1.0.0

**npm**: both 3.10.10 & 5.6.0

**node**: both 6.9.3 & 9.8.0

Start with the runValidator call at w3c-link-validator/src/w3clink.js:70 and reproduce the failure using `w3clink check http://localhost:8000`. Trace why the request callback reaches `path.basename` with null, then verify that the command completes without the TypeError for the provided reproduction case.

Written by the indexing model from the issue text.