fix: force one pairlist handler via config

closes #12112

build_helpers/schema.json

@@ -572,6 +572,7 @@
     "pairlists": {
       "description": "Configuration for pairlists.",
       "type": "array",
+      "minItems": 1,
       "items": {
         "type": "object",
         "properties": {

freqtrade/config_schema/config_schema.py

@@ -453,6 +453,7 @@ CONF_SCHEMA = {
         "pairlists": {
             "description": "Configuration for pairlists.",
             "type": "array",
+            "minItems": 1,
             "items": {
                 "type": "object",
                 "properties": {
@@ -1381,6 +1382,7 @@ SCHEMA_TRADE_REQUIRED = [
     "entry_pricing",
     "stoploss",
     "minimal_roi",
+    "pairlists",
     "internals",
     "dataformat_ohlcv",
     "dataformat_trades",
@@ -1390,6 +1392,7 @@ SCHEMA_BACKTEST_REQUIRED = [
     "exchange",
     "stake_currency",
     "stake_amount",
+    "pairlists",
     "dry_run_wallet",
     "dataformat_ohlcv",
     "dataformat_trades",

freqtrade/configuration/configuration.py

@@ -84,9 +84,6 @@ class Configuration:
         if "internals" not in config:
             config["internals"] = {}
 
-        if "pairlists" not in config:
-            config["pairlists"] = []
-
         # Keep a copy of the original configuration file
         config["original_config"] = deepcopy(config)