Looker Studio won't treat number as text

MSSQL VARCHAR field “job_number” contains alphanumeric & numeric rows (e.g. C001456, 240014) as well as NULL. Looker Studio has it as a TEXT field

I’m trying to create a calculated field based on this field based on the first character. If

SUBSTR(job_number, 0, 2)) Returns blank if the row is numeric

SUBSTR(CAST(job_number AS TEXT), 0, 1) Returns blank if the row is numeric

SUBSTR(CONCAT(job_number, “”), 0, 2) Returns blank if the row is numeric

LEFT_TEXT(CAST(job_number AS TEXT),2) Returns blank if the row is numeric