如果R中的所有值都是NA，如何 Select 具有最大值的行

我有一个数据框，看起来像这样:

tt1 <- structure(list(sjlid = c("SJL1527107", "SJL1527107", "SJL1527107", 
"SJL1527107", "SJL1527107"), condition = c("Abnormal_glucose_metabolism", 
"Abnormal_glucose_metabolism", "Abnormal_glucose_metabolism", 
"Abnormal_glucose_metabolism", "Abnormal_glucose_metabolism"), 
    grade = c(NA, NA, NA, NA, NA), ageevent = c(58.8352421588442, 
    62.1366120218579, 64.4872969533648, 68.9694887341867, 70.9612695561045
    )), row.names = 72:76, class = "data.frame")

我需要运行以下代码:

library(dplyr)
tt1 %>% group_by(condition) %>% top_n(1, grade) %>% top_n(1, ageevent)

当有多行(如tt1)时，该代码可以工作，但如果只有一行(如下面的tt2)，它就无法给出特定的行.例如，如果我的数据框是这样的:

tt2 <- structure(list(sjlid = "SJL1527107", condition = "Abnormal_glucose_metabolism", 
    grade = NA, ageevent = 58.8352421588442), row.names = 72L, class = "data.frame")

tt2 %>% group_by(condition) %>% top_n(1, grade) %>% top_n(1, ageevent)一无所获

# A tibble: 0 x 4
# Groups:   condition [0]
# ... with 4 variables: sjlid <chr>, condition <chr>, grade <lgl>, ageevent <dbl>

相反，我希望它返回这一行，因为这是唯一一行.

        sjlid                   condition grade         ageevent
72 SJL1527107 Abnormal_glucose_metabolism    NA 58.8352421588442