Fixes variable values of non-null type with default value (#2191)

Fixes #2190

Author: IvanGoncharov

src/execution/__tests__/variables-test.js

@@ -577,6 +577,20 @@ describe('Execute: Handles inputs', () => {
   });
 
   describe('Handles non-nullable scalars', () => {
+    it('allows non-nullable variable to be omitted given a default', () => {
+      const result = executeQuery(`
+        query ($value: String! = "default") {
+          fieldWithNullableStringInput(input: $value)
+        }
+      `);
+
+      expect(result).to.deep.equal({
+        data: {
+          fieldWithNullableStringInput: '"default"',
+        },
+      });
+    });
+
     it('allows non-nullable inputs to be omitted given a default', () => {
       const result = executeQuery(`
         query ($value: String = "default") {

src/execution/values.js

@@ -96,9 +96,7 @@ function coerceVariableValues(
     if (!hasOwnProperty(inputs, varName)) {
       if (varDefNode.defaultValue) {
         coercedValues[varName] = valueFromAST(varDefNode.defaultValue, varType);
-      }
-
-      if (isNonNullType(varType)) {
+      } else if (isNonNullType(varType)) {
         const varTypeStr = inspect(varType);
         onError(
           new GraphQLError(