Value Base64 should be a pointer to allow empty blobs

[Sadzeih]

While trying to insert an empty slice in a blob, I kept getting the same error;

missing field `base64`

While reading the library code, I noticed that if Base64 was and empty string "", due to omitempty, it was not marshalled.

Which caused the issue.

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The fix is to simply make Base64 into a pointer. That means for the blob type, the pointer will not be nil and will be marshalled.

[haaawk]

Thank you for your contribution @Sadzeih