fix(gatsby-image): ensure that currentSrc exists

[thebuilder]

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.

The issue was introduced by #12468 - it's simply missing an extra guard to ensure the value is defined.

[wardpeet]

Thanks for fixing this regression! works like a charm!

[wardpeet]

I'm on ie11 and I don't get this issue as it's wrapped inside IOSupported which is never true on ie11. Nonetheless, it's an ok check

[gatsbot]

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Thanks again!

[thebuilder]

I'm on ie11 and I don't get this issue as it's wrapped inside IOSupported which is never true on ie11. Nonetheless, it's an ok check

The error i caught: https://sentry.io/share/issue/da7dc5b5859943e09889b2089f52ee3b/

There's a lot of moving parts, in gatsby-image - Most likely the issue is triggered by the stack i'm using it with, combining Contentful with an ObjectFit polyfill.