Fixed handling SVG files with missing tag

[daniellockyer]

fix https://linear.app/tryghost/issue/SLO-151/[ghost]-cannot-read-properties-of-null-reading-attributes-an

• in the event the file doesn't contain a tag, the code currently crashes because it tries to read attributes fromundefined`
• we can fix that by checking the first element exists before reading from it
• also includes a breaking test