marker.hasMarkup does not detect markup properly for bold/italic buttons

[bantic]

As of 6d4983d:

• select text, click "B"
• type additional text inside the bold text
• select the bolded text and notice that "B" is not active (blue). It should be active.

This is because when text is input the post parser reparses that section and we end up re-creating the marker w/ the bold markup in the post parser's markupFromNode method. This skips the builder's marker cache because of two bugs there:

• attributes is present but empty ([]) — this line should check whether it is empty, not just absent
• newly-created markers without attributes are (mistakenly) not getting cached