It was the biggest night of Ryan Gosling’s career at the 96th annual Academy Awards, where the Barbie star was nominated for Best Support Actor — and brought down the house with a rousing rendition of I’m Just Ken. While Hollywood heavyweights congratulated him after his history-making performance, one person was notably absent: his wife, Eva Mendes. Insiders claim while Mendes, 50, was backstage at the award show, her public absence of support for her husband pointed to “trouble” in the marriage, RadarOnline.com has learned. “People were surprised Eva wasn’t by Ryan’s side,” a source said, ac…