printSchema: support empty descriptions (#2177)

Author: IvanGoncharov

Diff for: src/utilities/__tests__/schemaPrinter-test.js

@@ -483,6 +483,20 @@ describe('Type System Printer', () => {
     `);
   });
 
+  it('Prints an empty description', () => {
+    const output = printSingleFieldSchema({
+      type: GraphQLString,
+      description: '',
+    });
+
+    expect(output).to.equal(dedent`
+      type Query {
+        """"""
+        singleField: String
+      }
+    `);
+  });
+
   it('One-line prints a short description', () => {
     const description = 'This field is awesome';
     const output = printSingleFieldSchema({

Diff for: src/utilities/buildASTSchema.js

@@ -475,7 +475,7 @@ function getLeadingCommentBlock(node): void | string {
     comments.push(value);
     token = token.prev;
   }
-  return comments.reverse().join('\n');
+  return comments.length > 0 ? comments.reverse().join('\n') : undefined;
 }
 
 /**

Diff for: src/utilities/schemaPrinter.js

@@ -317,7 +317,7 @@ function printDescription(
   indentation = '',
   firstInBlock = true,
 ): string {
-  if (!def.description) {
+  if (def.description == null) {
     return '';
   }