fix(gatsby-image): ensure that currentSrc exists (gatsbyjs#13287)

* Ensure that currentSrc exists

Trying to read `currentSrc` before it has been set throws an `Unable to get property 'length' of undefined or null reference` error.
It should be an empty string, but IE11 can return `undefined`.

* Check if the `currentSrc` has a value

Instead of reading the `length`, we can just check to see if it's a value. This ensures it's not `undefined` and that it has a valid `src` URL

packages/gatsby-image/src/index.js

@@ -224,7 +224,7 @@ class Image extends React.Component {
         this.setState({ isVisible: true }, () =>
           this.setState({
             imgLoaded: imageInCache,
-            imgCached: this.imageRef.current.currentSrc.length > 0,
+            imgCached: !!this.imageRef.current.currentSrc,
           })
         )
       })