vulncheck: skip building call graph if there are no vuln packages
vulncheck: skip building call graph if there are no vuln packages

There is no need to search if there are any reachable vulnerable
functions if there aren't even any vulnerable packages imported.

Change-Id: Ic09cb6eed5fbb92e43696c62616de51284533dfb
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/417456
Run-TryBot: Zvonimir Pavlinovic <490af24f3def1258d2f7e9b10b20e341f6c68320@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>

golang/vuln,golang/vuln