TypeError: cannot read property 'split' of undefined

Hi,

version: 1.1.1

I am using Karma-coverage to generate lcov report with in-memory report like this:
```
coverageReporter: {
dir: 'reports/',
reporters: [
{ type: 'in-memory' },
{ type: 'lcov', subdir: 'report-lcov' },
]
},
```
I am getting the error:
```
TypeError: Cannot read property 'split' of undefined
at HtmlReport.writeDetailPage ({path}\node_modules\istanbul\lib\report\html.js:412:28)
```
I tried modifying the html.js to see the result. coverage is sending undefined value to istanbul in the end.
My reports are generating properly, but this error fails my build.
Is there a fix to it?

Start with the reported failure in istanbul/lib/report/html.js at HtmlReport.writeDetailPage, then trace the in-memory and lcov reporter paths from the supplied coverageReporter configuration. Reproduce the build with Karma-coverage 1.1.1 and inspect where the undefined value reaches Istanbul. Done means the reports still generate without the split error and the build succeeds.

Written by the indexing model from the issue text.