tools(i): Enable nilness linter (#2383)

## Relevant issue(s)

Resolves #2382 

## Description

This PR enables the `nilness` linter and fixes a few issues caught by
it.

## Tasks

- [x] I made sure the code is well commented, particularly
hard-to-understand areas.
- [x] I made sure the repository-held documentation is changed
accordingly.
- [x] I made sure the pull request title adheres to the conventional
commit style (the subset used in the project can be found in
[tools/configs/chglog/config.yml](tools/configs/chglog/config.yml)).
- [x] I made sure to discuss its limitations such as threats to
validity, vulnerability to mistake and misuse, robustness to
invalidation of assumptions, resource requirements, ...

## How has this been tested?

`make lint`

Specify the platform(s) on which this was tested:
- MacOS

http/handler_lens.go

@@ -88,10 +88,6 @@ func (s *lensHandler) MigrateDown(rw http.ResponseWriter, req *http.Request) {
 		responseJSON(rw, http.StatusBadRequest, errorResponse{err})
 		return
 	}
-	if err != nil {
-		responseJSON(rw, http.StatusBadRequest, errorResponse{err})
-		return
-	}
 	var value []map[string]any
 	err = enumerable.ForEach(result, func(item map[string]any) {
 		value = append(value, item)

request/graphql/parser/request.go

@@ -48,7 +48,7 @@ func ParseRequest(schema gql.Schema, doc *ast.Document) (*request.Request, []err
 			}
 
 			parsedDirectives, err := parseDirectives(astOpDef.Directives)
-			if errs != nil {
+			if err != nil {
 				return nil, []error{err}
 			}
 			parsedQueryOpDef.Directives = parsedDirectives

tools/configs/golangci.yaml

@@ -287,6 +287,7 @@ linters-settings:
     # run `go tool vet help` to see all analyzers
     enable:
       - atomicalign
+      - nilness
 
     enable-all: false