v3 - plan cost will not be shown when unit is empty string

[#172146647](https://www.pivotaltracker.com/story/show/172146647)

Author: FelisiaM

app/presenters/v3/service_plan_presenter.rb

@@ -148,7 +148,8 @@ def costs_schema
                   }
                 },
                 'unit' => {
-                  'type' => 'string'
+                  'type' => 'string',
+                  'minLength' => 1
                 }
               }
             }

spec/unit/presenters/v3/service_plan_presenter_spec.rb

@@ -317,6 +317,24 @@
                 }
             ]
           }'
+        ],
+        ['unit is empty string',
+         '{
+             "costs": [
+                {
+                  "amount": {
+                    "gpb": 0.06
+                  },
+                  "unit": "Daily"
+                },
+                {
+                  "amount": {
+                    "usd": 0.10
+                  },
+                  "unit": ""
+                }
+            ]
+          }'
         ]
       ].each do |scenario, extra|
         it "returns empty cost array when #{scenario}" do