Sparkr 我如何才能保留NA'；使用dapply时的时间戳

我正在尝试使用dapply将许多日期字符列转换为时间戳格式。但是，将空字符行转换为起始日期“1970-01-01”

df
df <- data.frame(a = c("12/31/2016", "12/31/2016", "12/31/2016"), 
             b = c("01/01/2016", "01/01/2017", ""))

ddf <- as.DataFrame(df)

schema <- structType(
    structField("a", 'timestamp'),
    structField("b", 'timestamp'))

converted_dates <- dapply(ddf, 
            function(x){ as.data.frame(lapply(x, function(y) as.POSIXct(y, format = "%m/%d/%Y"))) },
            schema)

 head(converted_dates)
       a          b
1 2016-12-31 2016-01-01
2 2016-12-31 2017-01-01
3 2016-12-31 1970-01-01

as.data.frame(lapply(df, function(y) as.POSIXct(y, format = "%m/%d/%Y")))
       a          b
1 2016-12-31 2016-01-01
2 2016-12-31 2017-01-01
3 2016-12-31       <NA>