fix(plugin): nilness findings (#7556)

Enable nilness linter in govet.

Plugin-by-plugin rationale:

- plugin/transfer: reuse error instead of shadowing it inside the for
  loop by declaring "ret" outside of the loop
- plugin/view: remove redundant err check
- plugin/dnstap: avoid possible nil dereference in error reporting
  path in setup test
- plugin/forward: prevent nil deference or empty-slice dereference on
  error paths in setup test

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

.golangci.yml

@@ -31,6 +31,9 @@ linters:
       - legacy
       - std-error-handling
   settings:
+    govet:
+      enable:
+        - nilness
     revive:
       rules:
         - name: blank-imports