Do not send a quota guid if no quota is specified.

The runtime has added more checks to prevent bad quotas.  Sending a "" is caught as an invalid quota name.
Sending nothing is the right behavior now.
Looks like there aren't too many more instances of this "pattern" to fix.

```
tjarratt at RetinaBook in ~/git/cf-cli on master
± ack "Sprintf\(\`{"
cf/api/feature_flags/feature_flags.go
58:	body := fmt.Sprintf(`{"enabled": %v}`, set)

cf/api/organizations/organizations.go
73:	data := fmt.Sprintf(`{"name":"%s", "quota_definition_guid":"%s"}`, org.Name, org.QuotaDefinition.Guid)
79:	data := fmt.Sprintf(`{"name":"%s"}`, name)

cf/api/password.go
34:	body := fmt.Sprintf(`{"password":"%s","oldPassword":"%s"}`, new, old)

cf/api/quotas/quotas.go
84:	data := fmt.Sprintf(`{"quota_definition_guid":"%s"}`, quotaGuid)

cf/api/routes.go
70:	data := fmt.Sprintf(`{"host":"%s","domain_guid":"%s","space_guid":"%s"}`, host, domainGuid, spaceGuid)

cf/api/service_auth_tokens.go
70:	body := fmt.Sprintf(`{"label":"%s","provider":"%s","token":"%s"}`, authToken.Label, authToken.Provider, authToken.Token)
81:	body := fmt.Sprintf(`{"token":"%s"}`, authToken.Token)

cf/api/service_brokers.go
91:	body := fmt.Sprintf(`{"name":"%s"}`, name)

cf/api/service_plan.go
34:	body = fmt.Sprintf(`{"name":"%s", "free":%t, "description":"%s", "public":%t, "service_guid":"%s"}`,

cf/api/service_plan_visibility.go
34:	data := fmt.Sprintf(`{"service_plan_guid":"%s", "organization_guid":"%s"}`, serviceGuid, orgGuid)

cf/api/services.go
155:	body := fmt.Sprintf(`{"name":"%s"}`, newName)
245:	body := strings.NewReader(fmt.Sprintf(`{"service_plan_guid":"%s"}`, v2PlanGuid))

cf/api/spaces/spaces.go
70:	body := fmt.Sprintf(`{"name":"%s","organization_guid":"%s"}`, name, orgGuid)
82:	body := fmt.Sprintf(`{"name":"%s"}`, newName)
```