Empty donation amount is NaN, causing invalid responses from the payment gateways

[mal-tee]

If you donate NaN with crypto you get this error message.

Debug information:

{
    "message": "Critical: Malformed response body when trying to get payment provider auth URL.",
    "type": "error",
    "colno": 46488,
    "filename": "https://www.datenanfragen.de/js/general.gen.js",
    "lineno": 20,
    "error": {
        "code": 2,
        "message": "Malformed response body when trying to get payment provider auth URL.",
        "name": "Critical",
        "stack": "n@https://www.datenanfragen.de/js/general.gen.js:20:46488\nt@https://www.datenanfragen.de/js/general.gen.js:20:41448\nt@https://www.datenanfragen.de/js/general.gen.js:13:17040\nt@https://www.datenanfragen.de/js/general.gen.js:13:17380\nt/</<@https://www.datenanfragen.de/js/general.gen.js:20:445505\n",
        "enduser_message": "Ungültige Antwort von unserem Zahlungsdienstleister. Bitte versuche es erneut.",
        "context": {
            "donation_reference": "200501XBQTDB51K",
            "request": {
                "payment_provider": "coingate",
                "data": {
                    "price_amount": "NaN",
                    "price_currency": "EUR",
                    "receive_currency": "EUR",
                    "title": "Spende 200501XBQTDB51K",
                    "success_url": "https://www.datenanfragen.de/donate?donation_reference=200501XBQTDB51K",
                    "cancel_url": "https://www.datenanfragen.de/donate",
                    "order_id": "200501XBQTDB51K"
                }
            },
            "response": {
                "message": "CoinGate payment initiation failed."
            }
        }
    },
    "code_version": "1.0.0",
    "user_agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:75.0) Gecko/20100101 Firefox/75.0",
    "url": {
        "href": "https://www.datenanfragen.de/donate/",
        "origin": "https://www.datenanfragen.de",
        "protocol": "https:",
        "host": "www.datenanfragen.de",
        "hostname": "www.datenanfragen.de",
        "port": "",
        "pathname": "/donate/",
        "search": "",
        "hash": ""
    }
}

[baltpeter]

Thanks for the report! Should be fixed by #301, once that is merged.