You are given a table of defect records from a BAE Systems USA engineering tracking workflow. Write a SQL query to return the number of open defects for each severity, sorted by the defect count from highest to lowest. Ignore rows where severity is NULL.
| Column | Type | Description |
|---|---|---|
| defect_idPK | INT | Unique defect record ID |
| program_name | VARCHAR(100) | Program or platform name |
| severity | VARCHAR(20) | Defect severity level |
| status | VARCHAR(20) | Current defect status |
| reported_date | DATE | Date the defect was reported |