The first_value(… ignore nulls) window function with ignore nulls inside the parentheses is not standard SQL. It evaluates its argument in the rows defined by the over clause in order and returns the first non-null value (if any).
SELECT FIRST_VALUE(<expr> IGNORE NULLS) OVER (ORDER BY …)
FROM …Related
Same functionality in standard SQL:
first_value(…) ignore nulls
Normative References
Ignore nulls inside the parentheses of first_value() is not covered in the international SQL standard ISO/IEC 9075:2016-2. In standard SQL the ignore nulls clause follows the parentheses: first_value() ignore nulls.
