Make getElementById Conditional (#6145)

Firefox shows this warning...
Empty string passed to getElementById()

Author: jigz

packages/gatsby-link/src/index.js

@@ -137,7 +137,7 @@ class GatsbyLink extends React.Component {
                 .split(`#`)
                 .slice(1)
                 .join(`#`)
-              const element = document.getElementById(hashFragment)
+              const element = hashFragment ? document.getElementById(hashFragment) : null
               if (element !== null) {
                 element.scrollIntoView()
                 return true