Twenty Ten: Removed header image renders an empty img tag

[koopersmith]

When the header image is removed from Twenty Ten, the theme still renders an img tag with no src (and the proper dimensions, so it's just a large, broken image). Instead, the img tag shouldn't be rendered.

[nacin]

(In [15520]) Check for header image before showing it, in Twenty Ten. props koopersmith, see #14486 for trunk.

[nacin]

(In [15783]) Check for header image before showing it, in Twenty Ten. props koopersmith, fixes #14486 for 3.0.