/find results screen crashes on a shop with null price(s)

/find <item> throws in the results screen and the whole dialog fails:

java.lang.IllegalArgumentException: Cannot format given Object as a Number
    at java.text.DecimalFormat.format(...)
    at io.paradaux.chestshop.dialogs.FindDialog.buyPart(FindDialog.java:306)
    at io.paradaux.chestshop.dialogs.FindDialog.priceSummary(FindDialog.java:297)
    at io.paradaux.chestshop.dialogs.FindDialog.shopLabel(FindDialog.java:278)
    at io.paradaux.chestshop.dialogs.FindDialog.results(FindDialog.java:219)

Root cause: FoundShop.buyPriceValue()/sellPriceValue() return a boxed Double that is null when the corresponding price is null. DecimalFormat.format(Object) throws "Cannot format given Object as a Number" on null (null instanceof Number is false). FoundShop.shopType() falls through to BUY when both prices are null, so priceSummary calls buyPart → MONEY.format(null) → crash. Any registry row with both prices null kills the results screen.

Fix: made priceSummary presence-based — it formats only the side(s) that actually have a non-null price, renders no price when neither is present, and no longer trusts shopType()'s BUY fallback. dialogs/** is excluded from the coverage gate, so no unit test is required.