Null entries in sourcesContent

[glromeo]

I ran into an issue that was caused by sourceMaps having null values in the array of sourcesContent causing:

v8-to-istanbul/lib/source.js

Line 6 in 53c1cd8

sourceRaw = sourceRaw.trimEnd()

...to throw an error.

This PR aims at filtering these null values which is consistent with the behaviour of :

v8-to-istanbul/lib/v8-to-istanbul.js

Lines 203 to 205 in 53c1cd8

	const matchingSource = this.covSources.find(covSource => covSource.path === source)
	covSource = matchingSource ? matchingSource.source : this.covSources[0].source
	path = matchingSource ? matchingSource.path : this.covSources[0].path

...of ignoring source contents that might be missing

NOTE: This PR is meant to be merged after #173

[bcoe]

@glromeo it looks like these changes were included in the perf fix we just landed.