What does “Empty dress” exactly mean?

Washington Post (May 22) reported the victory of the 35-year-old Alison Lundergan Grimes in Kentucky Democratic primary to position her as the challenger to 72-year-old Senate’s GOP leader, Mitch McConnell in November election.

It says;

Alison Lundergan Grimes says it everywhere she goes. She said it at dozens of stops in Kentucky over the past week. She said it at her victory speech here Tuesday night after securing the Democratic nomination for Senate. And she plans to say it again all the way to November. She’s not an “empty dress.”

“I am not an empty dress, I am not a rubber stamp, and I am not a cheerleader,” she said in a speech Tuesday night after she and McConnell each easily defeated primary opponents and officially began what is shaping up to be one of the year’s most heated political battles. - Source

None of OED, CED, Merriam-Webster carries “empty dress.” Google Ngram shows existence of the word since mid 19 century, but at a very low incidence ratio (0.0000002% in 2008)

Urban dictionary defines it as “Usually used in regards a woman. After you get past all the glamour and glitz, there is nothing worth staying for.”

Does it mean ‘outdated and unattractive woman without substance’? What is an alternative short word to “empty dress” used in the context of Alison Grimes’ victory speech? What is the men’s version of “empty dress”?

The meaning is to be of no substance. She gave the meaning fairly well when she amplified it by saying she was not a rubber stamp or a cheerleader. Sometimes a man is demeaned in a similar way by saying he is an empty suit, or "all hat and no cattle."

The following quote from MSNBC should give you some context:

“Alison Lundergan Grimes seems incapable of articulating her own thoughts, and faced with questions, either directly parrots the talking points handed to her by [Sen.] Chuck Schumer or she babbles incoherently and stares blankly into the camera as though she’s a freshman in high school struggling to remember the CliffsNotes after forgetting to read her homework assignment,” NRSC Communications Director Brad Dayspring said in an email to The Hill. “They say you can’t be something with nothing, but Alison Lundergan Grimes seems determined to test that theory. She’s an empty dress.”

Brad Dayspring is essentially stating that Alison Lundergan has no substance, hence the empty dress.