Merge #445

445: allow null values in csv r=Kerollmops a=MarinPostma

This pr allows null values in csv:
- if the field is of type string, then an empty field is considered null (`,,`), anything other is turned into a string (i.e `, ,` is a single whitespace string)
- if the field is of type number, when the trimmed field is empty, we consider the value null (i.e `,,`, `, ,` are both null numbers) otherwise we try to parse the number.


Co-authored-by: ad hoc <postma.marin@protonmail.com>

milli/src/documents/builder.rs

@@ -118,7 +118,10 @@ impl<W: io::Write + io::Seek> DocumentBatchBuilder<W> {
             for (value, (fid, ty)) in record.into_iter().zip(headers.iter()) {
                 let value = match ty {
                     AllowedType::Number => {
-                        value.parse::<f64>().map(Value::from).map_err(|error| {
+                        if value.trim().is_empty() {
+                            Value::Null
+                        } else {
+                            value.trim().parse::<f64>().map(Value::from).map_err(|error| {
                                 Error::ParseFloat {
                                     error,
                                     // +1 for the header offset.
@@ -127,7 +130,14 @@ impl<W: io::Write + io::Seek> DocumentBatchBuilder<W> {
                                 }
                             })?
                         }
-                    AllowedType::String => Value::String(value.to_string()),
+                    }
+                    AllowedType::String => {
+                        if value.is_empty() {
+                            Value::Null
+                        } else {
+                            Value::String(value.to_string())
+                        }
+                    }
                 };
 
                 this.value_buffer.clear();