fix: custom analyzer converter none severity case

Author: Artur-Wisniewski

packages/custom_lint_builder/lib/src/custom_analyzer_converter.dart

@@ -81,20 +81,28 @@ class CustomAnalyzerConverter {
     for (final error in errors) {
       final processor = analyzer.ErrorProcessor.getProcessor(options, error);
       final configSeverity = configSeverities?[error.errorCode.name];
-
       // Config severities override processor severities
       final severity = configSeverity ?? processor?.severity;
-
-      // Errors with null severity or NONE are filtered out.
-      if (severity == null || severity == analyzer.ErrorSeverity.NONE) {
+      if (severity == analyzer.ErrorSeverity.NONE) {
         continue;
       }
 
-      serverErrors.add(convertAnalysisError(
-        error,
-        lineInfo: lineInfo,
-        severity: severity,
-      ));
+      if (processor != null) {
+        // Errors with null severity are filtered out.
+        if (severity != null) {
+          serverErrors.add(convertAnalysisError(
+            error,
+            lineInfo: lineInfo,
+            severity: severity,
+          ));
+        }
+      } else {
+        serverErrors.add(convertAnalysisError(
+          error,
+          lineInfo: lineInfo,
+          severity: severity,
+        ));
+      }
     }
     return serverErrors;
   }