fix(filter): "equals nothing" found nothing

A NULL never equals '', so a condition written that way returned zero rows. The
question is perfectly clear — the operator wants the ones with nothing in that
field — and answering it with silence makes the filter look broken rather than
mis-stated. eq and ne with a blank value now run the empty / not-empty test.

Empty on a NUMERIC column also had a real fault behind it. IFNULL(col,'')=''
compares 0 against '', which SQLite calls false and MySQL calls true — one
expression quietly answering two different questions depending on where the
logbook lives. Numeric columns test NULL or zero explicitly now; text keeps the
string test, where '' is a real value and 0 is not.

internal/qso/qso.go

@@ -1266,6 +1266,17 @@ var filterableColumns = map[string]bool{
 // value compares on the date part (see conditionSQL) so day filters are exact.
 var dateColumns = map[string]bool{"qso_date": true, "qso_date_off": true}
 
+// numericColumns are the filterable columns holding numbers rather than text.
+//
+// "Empty" means something different for them: NULL *or* zero. It has to be said
+// explicitly because the two backends disagree — SQLite compares 0 against ”
+// as false, MySQL calls it true — so one expression would quietly answer two
+// different questions depending on where the logbook lives.
+var numericColumns = map[string]bool{
+	"freq_hz": true, "freq_rx_hz": true, "dxcc": true, "cqz": true, "ituz": true,
+	"srx": true, "stx": true, "tx_pwr": true,
+}
+
 // bareDateRe matches a plain calendar date with no time component.
 var bareDateRe = regexp.MustCompile(`^\d{4}-\d{2}-\d{2}$`)
 
@@ -1366,6 +1377,18 @@ func conditionSQL(c Condition) (string, []any, error) {
 		col = "substr(" + col + ",1,10)"
 		v = strings.TrimSpace(v)
 	}
+	// "equals nothing" and "is empty" are the same question, and SQL answers the
+	// first with nothing at all: a NULL never equals '', so a filter written that
+	// way returns zero rows and looks broken rather than wrong. Asking it in
+	// plain words is not a mistake worth punishing.
+	if strings.TrimSpace(v) == "" {
+		switch c.Op {
+		case "eq":
+			c.Op = "empty"
+		case "ne":
+			c.Op = "notempty"
+		}
+	}
 	switch c.Op {
 	case "eq":
 		return col + " = ?", []any{v}, nil
@@ -1416,8 +1439,18 @@ func conditionSQL(c Condition) (string, []any, error) {
 		}
 		return col + " IN (" + ph + ")", args, nil
 	case "empty":
+		// A numeric column is empty when it is NULL *or* zero, and that has to be
+		// said explicitly: SQLite compares 0 against '' as false while MySQL calls
+		// it true, so IFNULL(col,'')='' quietly means different things on the two
+		// backends OpsLog supports.
+		if numericColumns[strings.ToLower(strings.TrimSpace(c.Field))] {
+			return "(" + col + " IS NULL OR " + col + " = 0)", nil, nil
+		}
 		return "IFNULL(" + col + ",'') = ''", nil, nil
 	case "notempty":
+		if numericColumns[strings.ToLower(strings.TrimSpace(c.Field))] {
+			return "(" + col + " IS NOT NULL AND " + col + " <> 0)", nil, nil
+		}
 		return "IFNULL(" + col + ",'') <> ''", nil, nil
 	default:
 		return "", nil, fmt.Errorf("unknown operator %q", c.Op)