Preserve Region type

[stephen-soltesz]

This change updates the logic that masks the Region field to preserve the type as STRING.

Previously, because NULL has no type, the schema is inferred as an INTEGER without explicit direction. This change updates the Region value to use CAST(NULL as STRING) to preserve the string type and the NULL value.

---

This change is 

[coveralls]

Pull Request Test Coverage Report for Build 660

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 71.123%

---

Totals
Change from base Build 652:	0.0%
Covered Lines:	133
Relevant Lines:	187

---

💛 - Coveralls