Fix: guard against undefined conversionValue

[ZLeventer]

Summary

• Fixes CAPI tag throws error if conversionValue is undefined #14 — JSON.parse(data.conversionValue) crashes when conversionValue is undefined
• Changed from JSON.parse(data.conversionValue) || { ... } to a ternary check: data.conversionValue ? JSON.parse(data.conversionValue) : { ... }
• The fallback object with currencyCode and amount is now used correctly when no conversion value is provided

Test plan

• Deploy tag to a GTM container without setting conversionValue
• Verify the tag fires without errors and uses the fallback currency/amount values
• Verify existing behavior is unchanged when conversionValue is provided