alwayse set schema url

Author: zeitlinger

exporters/prometheus/src/main/java/io/opentelemetry/exporter/prometheus/Otel2PrometheusConverter.java

@@ -487,9 +487,7 @@ private Labels convertAttributes(
         labelNameToValue.putIfAbsent(OTEL_SCOPE_VERSION, scope.getVersion());
       }
       String schemaUrl = scope.getSchemaUrl();
-      if (schemaUrl != null) {
-        labelNameToValue.putIfAbsent(OTEL_SCOPE_SCHEMA_URL, schemaUrl);
-      }
+      labelNameToValue.putIfAbsent(OTEL_SCOPE_SCHEMA_URL, schemaUrl == null ? "" : schemaUrl);
       scope
           .getAttributes()
           .forEach(