Fix NPE in Portfolio View when market price n/a

[ghost]

Query of market price may return null when the price is not available (e.g. priceserver temporarily down).
Portfolio View TriggerPriceService was not checking the market price object prior to use.

Fixes #5381

[wallclockbuilder]

Created an offer, then set marketPrice=null;
Tried viewing the offer.
Got this error in the UI and the following error log.

[JavaFX Application Thread] ERROR b.c.s.CommonSetup: Stack trace: java.lang.NullPointerException at bisq.core.offer.TriggerPriceService.wasTriggered(TriggerPriceService.java:125) at bisq.core.offer.TriggerPriceService.checkPriceThreshold(TriggerPriceService.java:140) at bisq.core.offer.TriggerPriceService.lambda$onPriceFeedChanged$4(TriggerPriceService.java:108) at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:183) at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:177) at java.base/java.util.HashMap$KeySpliterator.forEachRemaining(HashMap.java:1608)

I'm able to view the offer just fine when the null value is guarded.

[ripcurlx]

utACK