get_flow_data() fails if any gage has no data

[allopole]

get_flow_data() will fail if readNWISdv() returns no rows for a site, because bind_rows() does not know how to handle the empty dataframes. Proposed fix: skip dataframes with zero rows before binding rows. See last 5 lines below for the code fix.

get_flow_data <- function (gages_df, start_date, end_date) 
{
  if (!is.data.frame(gages_df)) 
    stop("gage_df must be a data.frame")
  flow_data <- lapply(gages_df$site_no, function(x) {
    dataRetrieval::readNWISdv(siteNumber = x, parameterCd = "00060", 
                              startDate = start_date, endDate = end_date)
  })
  omit <- sapply(flow_data, function(x) {
    length(x)
  })
  flow_data <- lapply(flow_data[omit > 0], function(x) {
    x
  })
  flow_data <- lapply(flow_data, function(x) {
    colnames(x)[3] <- "date"
    colnames(x)[4] <- "discharge"
    return(x)
  })
  skip <- which(unlist(lapply(flow_data, function(x) nrow(x))) == 0)
  flow_data <- dplyr::bind_rows(flow_data[-skip]) %>% dplyr::select_(~site_no, 
                                                              ~date, ~discharge) %>% dplyr::filter_(~discharge > 0)
  return(flow_data)
}