chemical_equal_batch() throws error if a CASRN doesn't return any results

Reproducible example:

` ctxR::chemical_equal_batch("16041-87-0")`

Resulting error:
```
Error in is.null(t) || is.na(t) : 
  'length = 2' in coercion to 'logical(1)'
```

Compare the output of `ctxR::chemical_equal("16041-87-0")`, which shows that this CASRN doesn't return any search results:

 ```
   Chemical Suggestion
1 16041-87-0         NA
```