mapPreview breaks when rule visibility is set to false

[darioh91]

Hi,

we noticed bug with mapPreview while a user sets visibility to false on a rule. If this option is set to false, all features (and all styles) are missing from the map.
The problem occurs on line

geostyler-openlayers-parser/src/OlStyleParser.ts

Line 849 in 8d5dd38

styles.push(null);

An array containing null values causes an error. The same issue appears at the line

geostyler-openlayers-parser/src/OlStyleParser.ts

Line 855 in 8d5dd38

styles.push(null);

It would probably be sufficient to return early from forEach loop in cases when rule.symbolizer has visibility set to false.