fix: do not allow empty string for port

Author: snitin315

lib/options.json

@@ -523,7 +523,8 @@
           "type": "number"
         },
         {
-          "type": "string"
+          "type": "string",
+          "minLength": 1
         },
         {
           "enum": ["auto"]

test/validate-options.test.js

@@ -258,8 +258,8 @@ const tests = {
     failure: ['', [], { foo: 'bar' }, { target: 90 }, { app: true }],
   },
   port: {
-    success: ['', 0, 'auto'],
-    failure: [false, null],
+    success: ['8080', 8080, 'auto'],
+    failure: [false, null, ''],
   },
   proxy: {
     success: [