`extendSchema()` doesn't set `GraphQLSchema.description` in some cases

[sachindshinde]

extendSchema() doesn't set GraphQLSchema.description if the schema definition is in the input GraphQLSchema instead of the AST.

Example
Running this code:

import { buildSchema, extendSchema, parse } from "graphql";

const extendedSchema = extendSchema(
  buildSchema(`
    "A schema description"
    schema {
      query: Foo
    }
    
    type Foo {
      foo: String
    }
  `),
  parse(`scalar Bar`),
);

console.log(`AST description: ${extendedSchema.astNode?.description?.value}`);
console.log(`GraphQLSchema description: ${extendedSchema.description}`);

will print this:

AST description: A schema description
GraphQLSchema description: undefined