Ensure empty YAML inputs yield checked exceptions

PiperOrigin-RevId: 682755056

Author: TristonianJones

policy/src/main/java/dev/cel/policy/BUILD.bazel

@@ -117,7 +117,6 @@ java_library(
         ":value_string",
         ":yaml_helper",
         "//common:compiler_common",
-        "//common:source_location",
         "//common/internal",
         "@maven//:com_google_guava_guava",
         "@maven//:org_yaml_snakeyaml",

policy/src/main/java/dev/cel/policy/CelPolicyYamlConfigParser.java

@@ -338,7 +338,13 @@ private CelPolicyConfig parseYaml(String source, String description)
         throws CelPolicyValidationException {
       Node node;
       try {
-        node = parseYamlSource(source);
+        Node yamlNode =
+            parseYamlSource(source)
+                .orElseThrow(
+                    () ->
+                        new CelPolicyValidationException(
+                            String.format("YAML document is malformed: %s", source)));
+        node = yamlNode;
       } catch (RuntimeException e) {
         throw new CelPolicyValidationException("YAML document is malformed: " + e.getMessage(), e);
       }

policy/src/main/java/dev/cel/policy/CelPolicyYamlParser.java

@@ -66,8 +66,15 @@ private static class ParserImpl implements PolicyParserContext<Node> {
 
     private CelPolicy parseYaml() throws CelPolicyValidationException {
       Node node;
+      String policySourceString = policySource.getContent().toString();
       try {
-        node = YamlHelper.parseYamlSource(policySource.getContent().toString());
+        Node yamlNode =
+            YamlHelper.parseYamlSource(policySourceString)
+                .orElseThrow(
+                    () ->
+                        new CelPolicyValidationException(
+                            String.format("YAML document is malformed: %s", policySourceString)));
+        node = yamlNode;
       } catch (RuntimeException e) {
         throw new CelPolicyValidationException("YAML document is malformed: " + e.getMessage(), e);
       }

policy/src/main/java/dev/cel/policy/YamlHelper.java

@@ -20,6 +20,7 @@
 import com.google.common.base.Joiner;
 import java.io.StringReader;
 import java.util.List;
+import java.util.Optional;
 import org.yaml.snakeyaml.LoaderOptions;
 import org.yaml.snakeyaml.Yaml;
 import org.yaml.snakeyaml.constructor.SafeConstructor;
@@ -68,10 +69,9 @@ public static boolean assertYamlType(
     return false;
   }
 
-  static Node parseYamlSource(String policyContent) {
+  static Optional<Node> parseYamlSource(String policyContent) {
     Yaml yaml = new Yaml(new SafeConstructor(new LoaderOptions()));
-
-    return yaml.compose(new StringReader(policyContent));
+    return Optional.ofNullable(yaml.compose(new StringReader(policyContent)));
   }
 
   static boolean assertRequiredFields(