You could format the cell as eg "#,##0_);(#,##0);" (no quotes).
If you want a true null value:
=IF(INDEX('AML101-ETCH'!B2:B36,MATCH('POLY ETCH CD DATA'!B6,'AML101-ETCH'!A2:A36,1))=0,"",INDEX('AML101-ETCH'!B2:B36,MATCH('POLY ETCH CD DATA'!B6,'AML101-ETCH'!A2:A36,1)))