Adds and passes test for blank and nil csv values

[nolantait]

Currently, calling a CSV template with blank or nil values fails when looking up the first byte:

# lib/phlex/csv.rb

if FORMULA_PREFIXES_MAP[value.getbyte(0)]

# ...

first_byte = value.getbyte(0)
if FORMULA_PREFIXES_MAP[first_byte]

Raises a confusing eval error: no implicit conversion from nil to integer

This PR adds and passes specs that match the options for stripping whitespace.

Its currently written to do either:

• "" for blank values if not stripping whitespace, Ruby parses them to ""
• Nothing when stripping whitespace, Ruby parses them it to nil