Tests break on travis even though it runs locally.

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Need to find the reason why travis keeps breaking although it runs locally without problem.

Here is a failing Travis build for reference:
https://travis-ci.org/CorrelAid/datenguideR/jobs/609598285?utm_medium=notification&utm_source=github_status

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"Fixed" this issue with following pull request: #17

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However, the original problem still exists. I just implemented an error handler to deal with this.

There are two (probably related) issues on Travis:

1. cbind doesn't add source_dat because it has zero rows
   

   datenguideR/R/helpers_call_results.R

   Line 142 in 3d5d346

   cbind(source_dat) %>%

2. this left_join doesn't work because api_results has zero rows
   

   datenguideR/R/helpers_call_results.R

   Line 200 in 3d5d346

   dplyr::left_join(meta_info, by = substat_name_)

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I implemented two temporary "fixes" for this:

1. Only add source_dat if it's not empty

   datenguideR/R/helpers_call_results.R

   Lines 139 to 143 in 3d5d346

   	## TODO: This is necessary because Travis (for some reason) sometimes thinks source_dat is empty
   	if (!(nrow(source_dat)==0)) {
   	final <- final %>%
   	cbind(source_dat) %>%
   	tibble::as_tibble()

2. Error handle the add_substat_info function

   datenguideR/R/dg_call.R

   Lines 179 to 180 in 3d5d346

   	## TODO: This is necessary because Travis (for some reason) sometimes treats API results as empty
   	add_substat_info <- purrr::possibly(add_substat_info, otherwise = NULL)

[lhehnke]

See #32.