Filter out empty elements from list col

Well, this reprex is specific to the test data, but may help point you in the right direction if tidier data is not possible

library(purrr)
test_data <- structure(list(duration = c(2624L, 3374L, 901L), 
                            precincts = list(structure(list(),
                                                       .Names = character(0), 
                                                       row.names = integer(0),
                                                       class = "data.frame"),
                                             structure(list(smartzoneName = c("Loc 1", "Loc 2",
                                                                              "Loc 1", "Loc 2", "Loc 1"),
                                                            duration = c(264L, 1329L, 968L, 474L, 359L),
                                                            timestamp = c(1545522428000, 1545522692000,  
                                                                          1545524021000, 1545524989000, 1545525463000),
                                                            smartzoneId = c(275L, 251L, 275L, 251L, 275L)), 
                                                       class = "data.frame", 
                                                       row.names = c(NA, 5L)), list())),
                       row.names = c(NA, -3L), 
                       class = "data.frame")
test_data %>% pluck(2)[2]
#> [[1]]
#>   smartzoneName duration    timestamp smartzoneId
#> 1         Loc 1      264 1.545522e+12         275
#> 2         Loc 2     1329 1.545523e+12         251
#> 3         Loc 1      968 1.545524e+12         275
#> 4         Loc 2      474 1.545525e+12         251
#> 5         Loc 1      359 1.545525e+12         275

Created on 2019-01-10 by the reprex package (v0.2.1)