fix(csv)!: infer null for empty column.

[kskalski]

Which issue does this PR close?

Closes #4903.

Rationale for this change

Correctly reflect the fact the data had no values

What changes are included in this PR?

• inference starts with state implying Null data type, then moves to more specific types
• csv reader can read columns with Null data type, expects it to be empty

Are there any user-facing changes?

Previously inference assumed Utf8 data type for empty column, which later on could accept non-empty values. Now Null type is inferred and thus the column won't accept any non-empty values unless user modifies schema to handle null case differently.