docsite check chokes if an empty markdown file exists

[ryan-blunden]

To reproduce, add an empty file with .md file extension, then run docsite check.

anic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x15927e7]

goroutine 1 [running]:
github.com/sourcegraph/docsite.(*Site).checkContentPage(0xc0001dee60, 0x0, 0x0, 0x0, 0x0)
        /Users/rb/Projects/docsite/check.go:69 +0x47
github.com/sourcegraph/docsite.(*Site).Check(0xc0001dee60, 0x1884680, 0xc0000ca000, 0x0, 0x0, 0x0, 0x100d329, 0x1d0edc0, 0x1eacd98, 0x0)
        /Users/rb/Projects/docsite/check.go:50 +0x704
main.init.0.func1(0xc0000c6010, 0x0, 0x0, 0x0, 0x0)
        /Users/rb/Projects/docsite/cmd/docsite/check.go:21 +0xb8
main.commander.run(0xc0003f60c0, 0x2, 0x2, 0xc0000be120, 0x16e2469, 0x7, 0xc00002ee40, 0xc0000c6010, 0x1, 0x1)
        /Users/rb/Projects/docsite/cmd/docsite/cmd.go:117 +0x302
main.main()
        /Users/rb/Projects/docsite/cmd/docsite/main.go:46 +0xee