Systemic API to handle NA

I notice that we don't have any functionality to systematically handle `NA` in data.

For example:

```r
plot_r <- reactive({
chunks_reset()

ANL <- merged_data()$data() # nolint
# at this point the developer should be able to make decision of what to do with rows with NA
# for example, to remove rows with NA
# and then there should be some UI module that will output a warning message
# letting user know that some rows were removed
validate_has_data(ANL, 10)
```

Each column could be treated separately as well, with their own rules of how to handle NA.

If we add this feature, all code below this chunk will not have to ever worry about NA edge cases that will cause it to fail.

Provenance:
```
Creator: junlue
```