ICU-22950 Do not initialize currency for NumberPropertyMapper when useCurrency is false

[nightelf3]

resolveCurrency() slows down simple number formatting requests like:

for (int i = 0; i < 9999; i++)
  u_snprintf_u(buff, 100, u"%d", i);

Since icu4c doesn't use currency there, we can skip its initialization.

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22950
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[markusicu]

I expanded the rest of this (long) function, searched for "currency", and found some code that uses these variables but is gated by conditions other than useCurrency.

• if (!properties.currencyUsage.isNull())
• if (exportedProperties != nullptr)

If we think we can guarantee that useCurrency is true there, then we should add U_ASSERT(useCurrency) there.
For the second one, maybe a default-constructed currency is fine if not useCurrency?
Otherwise, we may need to add a runtime check for useCurrency.

@sffc @mihnita @richgillam @eggrobin

[richgillam]

Markus is right; this is a concern.

For if (!properties.currencyUsage.isNull()), I wonder if this could just be changed to useCurrency?

For exportedProperties->currency, I agree that a default-constructed currency ought to be fine.

For if (precision.fType == Precision::PrecisionType::RND_CURRENCY), I wonder if we could change this to if (useCurrency && precision.fType == Precision::PrecisionType::RND_CURRENCY)?

[nightelf3]

Added an assertion to if (!properties.currencyUsage.isNull()), since useCurrency is true when !properties.currencyUsage.isNull()
Used if (useCurrency && precision.fType == Precision::PrecisionType::RND_CURRENCY), thank you!

[richgillam]

Looks good. Thanks for the changes!