make matches key under Selectors optional (#8898)

Author: mtoffl01

internal-api/src/main/java/datadog/trace/bootstrap/config/provider/stableconfig/Selector.java

@@ -21,7 +21,9 @@ public Selector(Object yaml) {
     Map map = (Map) yaml;
     origin = (String) map.get("origin");
     key = (String) map.get("key");
-    matches = Collections.unmodifiableList((List<String>) map.get("matches"));
+    List<String> rawMatches = (List<String>) map.get("matches");
+    matches =
+        rawMatches != null ? Collections.unmodifiableList(rawMatches) : Collections.emptyList();
     operator = (String) map.get("operator");
   }
 

internal-api/src/test/groovy/datadog/trace/bootstrap/config/provider/StableConfigParserTest.groovy

@@ -42,6 +42,12 @@ apm_configuration_rules:
       matches: ["mysvc"]
     configuration:
       KEY_FOUR: "ignored"
+  - selectors:
+    - origin: process_arguments
+      key: "-Darg1"
+      operator: exists
+    configuration:
+      KEY_FIVE: "ignored"
 """
     Files.write(filePath, yaml.getBytes())
     StableConfigSource.StableConfig cfg = StableConfigParser.parse(filePath.toString())