fix(GraphQL): don't update cacheMb if not specified by user (GRAPHQL-…

…888) (#7103) (#7106)

Fixes GRAPHQL-888.
Previously, if you ran this request:
```
$ curl -H "Content-Type: application/json" http://localhost:8080/admin -d '{"query": "mutation {config(input: {logRequest: false}){response {code message}}}"}'
```
Alpha logs would also print this:
```
I1205 22:22:51.684693 2681396 middlewares.go:178] GraphQL admin mutation. Name =  config
I1205 22:22:51.684724 2681396 config.go:38] Got config update through GraphQL admin API
I1205 22:22:51.684810 2681396 worker.go:138] Updating cacheMb to 0
```
Indicating that cacheMb was also updated, even it wasn't specified in the request.

This PR fixes this issue.

(cherry picked from commit c03c327)

# Conflicts:
#	graphql/admin/config.go

graphql/admin/config.go

@@ -28,7 +28,7 @@ import (
 )
 
 type configInput struct {
-	LruMB float64
+	LruMB *float64
 	// LogRequest is used to update WorkerOptions.LogRequest. true value of LogRequest enables
 	// logging of all requests coming to alphas. LogRequest type has been kept as *bool instead of
 	// bool to avoid updating WorkerOptions.LogRequest when it has default value of false.
@@ -43,8 +43,9 @@ func resolveUpdateConfig(ctx context.Context, m schema.Mutation) (*resolve.Resol
 		return resolve.EmptyResult(m, err), false
 	}
 
-	if input.LruMB > 0 {
-		if err = worker.UpdateLruMb(input.LruMB); err != nil {
+	// update LruMB only when it is specified by user
+	if input.LruMB != nil {
+		if err = worker.UpdateLruMb(*input.LruMB); err != nil {
 			return resolve.EmptyResult(m, err), false
 		}
 	}