Don't create findings when there is no HTTP message

Proposal

Findings will not be created if there is no associated HTTP Message.

Example

If there is no HTTP message in the Navigation, then Browserker panics with the following error:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0xb2c6a0]
goroutine 174 [running]:
gitlab.com/browserker/scanner/plugin/vulnerabilities.(*PassiveCheck).buildFinding(0xc0005122c0, 0xc00b0a6d20, 0x0)
	/go/builds/scanner/plugin/vulnerabilities/passive_check.go:83 +0x80
gitlab.com/browserker/scanner/plugin/vulnerabilities.(*PassiveCheck).Check(0xc0005122c0, 0x0)
	/go/builds/scanner/plugin/vulnerabilities/passive_check.go:67 +0xf4
gitlab.com/browserker/scanner/plugin/vulnerabilities.(*FindingService).CheckAll(0xc0003da190, 0xea7008)
	/go/builds/scanner/plugin/vulnerabilities/finding_service.go:69 +0x296
gitlab.com/browserker/scanner/crawler.(*BrowserkCrawler).Process(0xc000525ba0, 0xc0002ff2d0, {0xebc788, 0xc00000ad20}, 0xc00b16ad80, 0x1)
	/go/builds/scanner/crawler/crawler.go:49 +0x150
gitlab.com/browserker/scanner.(*Browserk).crawl(0xc0000fa380, 0xc00b1ff0e0)
	/go/builds/scanner/browserk_scanner.go:392 +0x543
gitlab.com/browserker/scanner.(*Browserk).processEntries(0xc0000fa380)
	/go/builds/scanner/browserk_scanner.go:332 +0x205
created by gitlab.com/browserker/scanner.(*Browserk).Init
	/go/builds/scanner/browserk_scanner.go:137 +0x3dc