SQL NULL Handling

Xylem builds water-treatment equipment, and a field-engineer dashboard queries sensor readings from pump stations. The query below is supposed to flag stations with a missing or offline flow-rate reading, but the numbers don't match what field engineers see on site.

CREATE TABLE pump_stations (
    station_id INT PRIMARY KEY,
    pump_name VARCHAR(80),
    location_note VARCHAR(120)  -- nullable, filled in only for stations with special access instructions
);

CREATE TABLE sensor_readings (
    reading_id INT PRIMARY KEY,
    station_id INT,
    flow_rate_gpm NUMERIC(8,2),  -- nullable when a sensor fails to report
    status VARCHAR(20),          -- nullable when a station has never checked in
    recorded_at TIMESTAMP
);

SELECT
    ps.pump_name || ' - ' || ps.location_note AS display_label,
    AVG(sr.flow_rate_gpm) AS avg_flow_rate,
    COUNT(*) AS reading_count
FROM pump_stations ps
JOIN sensor_readings sr ON sr.station_id = ps.station_id
WHERE sr.flow_rate_gpm = NULL
   OR sr.status <> 'Offline'
GROUP BY ps.pump_name, ps.location_note;

What is wrong with this query's handling of NULLs, and how would you fix it?