Cox model with no covariates

[CamDavidsonPilon]

df = pd.DataFrame({
    'T': [5, 3, 9, 8, 7, 4, 4, 3, 2, 5, 6, 7],
    'E': [1, 1, 1, 1, 1, 1, 0, 0, 1, 1, 1, 0],
})

cph = CoxPHFitter()
cph.fit(df, 'T', 'E')
cph.print_summary()
cph.predict_median(df)

fails

[CamDavidsonPilon]

I feel this is needed to support for users who wish to calculate the log-likelihood ratio test the "hard" way (computing it manually). I've added some kinda hacky if statements to solve this.