Skip emitting empty <pre> blocks in documentation

We sometimes have decl-less comments interspersed in headers. Avoid the
empty patch of blue when they get rendered.

Change-Id: I6668946d07349febd6d28bbe7d4fd8d2426adca9
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/64927
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Bob Beck <bbe@google.com>
Commit-Queue: Bob Beck <bbe@google.com>

util/doc.go

@@ -649,7 +649,7 @@ func generate(outPath string, config *Config) (map[string]string, error) {
           {{range .Comment}}
             <p>{{. | markupPipeWords | newlinesToBR | markupFirstWord | markupRFC}}</p>
           {{end}}
-          <pre>{{.Decl}}</pre>
+          {{if .Decl}}<pre>{{.Decl}}</pre>{{end}}
           </div>
         {{end}}
         </div>