Data labels are not visible on a packedbubble chart when exporting PNG or JPEG

# Expected behaviour
Data labels to be clearly visible on the bubbles

# Actual behaviour
Data labels are invisible

# Reproduction steps
Send the following chart config:
```
{
"chart": {
"type": "packedbubble"
},
"plotOptions": {
"packedbubble": {
"dataLabels": {
"enabled": true
}
}
},
"series": [
{
"data": [
1,
2,
3
]
}
]
}
```

## When exporting a SVG or PDF, the labels are shown correctly:
```
curl 'http://localhost:7801/' \
--data-raw '{"infile":"{\n \"chart\": {\n \"type\": \"packedbubble\"\n },\n \"exporting\": {\n \"allowHTML\": true\n },\n \"plotOptions\": {\n \"packedbubble\": {\n \"dataLabels\": {\n \"enabled\": true\n }\n }\n },\n \"series\": [\n {\n \"data\": [\n 1,\n 2,\n 3\n ]\n }\n ]\n}\n","width":false,"scale":"2","constr":"chart","type":"image/png","b64":true}'
```
## But when exporting to PNG or to JPEG:
```
curl 'http://localhost:7801/' \
--data-raw '{"infile":"{\n \"chart\": {\n \"type\": \"packedbubble\"\n },\n \"exporting\": {\n \"allowHTML\": true\n },\n \"plotOptions\": {\n \"packedbubble\": {\n \"dataLabels\": {\n \"enabled\": true\n }\n }\n },\n \"series\": [\n {\n \"data\": [\n 1,\n 2,\n 3\n ]\n }\n ]\n}\n","width":false,"scale":"2","constr":"chart","type":"image/svg+xml","b64":true}'
```