[BUG] Reduce of empty array with no initial value

[helgehatt]

Bug Description

Missing handling of empty dateStrings array. Should be a good first issue.

ghostfolio/apps/api/src/services/exchange-rate-data/exchange-rate-data.service.ts

Lines 79 to 81 in 00c2ede

	const lastDateString = dateStrings.reduce((a, b) => {
	return a > b ? a : b;
	});

TypeError: Reduce of empty array with no initial value
    at Array.reduce (<anonymous>)
    at ExchangeRateDataService.getExchangeRatesByCurrency (/home/helge/github/ghostfolio/dist/apps/api/webpack:/src/services/exchange-rate-data/exchange-rate-data.service.ts:79:42)
    at PortfolioCalculator.getCurrentPositions (/home/helge/github/ghostfolio/dist/apps/api/webpack:/src/app/portfolio/portfolio-calculator.ts:534:7)
    at PortfolioService.getPerformance (/home/helge/github/ghostfolio/dist/apps/api/webpack:/src/app/portfolio/portfolio.service.ts:1206:9)
    at PortfolioController.getPerformanceV2 (/home/helge/github/ghostfolio/dist/apps/api/webpack:/src/app/portfolio/portfolio.controller.ts:401:36)

To Reproduce

1. Import activities in a new foreign currency
2. Refresh portfolio page before currency data has been fetched