fix: use nullish coalescing for coverage directory paths

[vladjerca]

Situation:

I want to output everything in ./coverage this cannot be easily achieve at the moment. If the subdirectory is set to '', it will fallback to the browser-name value.

Alternative:

At the moment I'm doing the following to achieve the desired result on my project:

'html': {
    'directory': 'coverage',
    'subdirectory': () => '',
},
'lcovonly': {
    'directory': 'coverage',
    'subdirectory': () => '',
},

Solution:

Use nullish, this way the defaults will be used only when the values are undefined or null.

[erikbarke]

Hey, thanks ☺️