Never make a header with value: null

Empty string is valid, null is invalid.

Author: Phil Sturgeon

src/serializers/postman/v2.0/Serializer.js

@@ -620,7 +620,7 @@ methods.createHeaderFromParameter = (param) => {
     return { key, value: schema.enum[0] }
   }
 
-  return { key, value: null }
+  return { key, value: '' }
 }
 
 /**

src/serializers/postman/v2.0/__tests__/Serializer.spec.js

@@ -844,7 +844,7 @@ describe('serializers/swagger/v2.0/Serializer.js', () => {
       ]
       const expected = [
         null,
-        { key: 123, value: null },
+        { key: 123, value: '' },
         { key: 234, value: 'abc' },
         { key: 345, value: 'def' }
       ]