fix rate source init panic

[upright39]

Config file (/home/upright/.cryptopower/cryptopower.conf) does not exist. Using defaults.
2023-10-23 18:14:18.043 [INF] DLWL: Starting check and remove all dir of deleted wallets....
2023-10-23 18:14:18.043 [ERR] DLWL: can't read DCR root wallet type: open /home/upright/.cryptopower/mainnet/dcr: no such file or directory
2023-10-23 18:14:18.043 [INF] DLWL: Loaded 0 wallets
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x10c2a8e]

goroutine 1 [running]:
github.com/crypto-power/cryptopower/libwallet.(*AssetsManager).GetCurrencyConversionExchange(...)
        /home/upright/Documents/cryptopower/libwallet/assets_config.go:141
github.com/crypto-power/cryptopower/libwallet.(*AssetsManager).initRateSource(0xc00022d780)
        /home/upright/Documents/cryptopower/libwallet/assets_manager.go:193 +0x10e
github.com/crypto-power/cryptopower/libwallet.NewAssetsManager({0xc00004d800?, 0x7?}, {0x182afc7, 0x3}, {0x185ada9, 0x20}, {0xc000412c00, 0x27}, {0x18300fd, 0x7})
        /home/upright/Documents/cryptopower/libwallet/assets_manager.go:172 +0x6c5
github.com/crypto-power/cryptopower/wallet.(*Wallet).InitAssetsManager(0xc00022d700)
        /home/upright/Documents/cryptopower/wallet/wallet.go:81 +0xbb
main.main()
        /home/upright/Documents/cryptopower/main.go:77 +0x32c
exit status 2