Replace Stream usage with iteration to avoid non-null requirements.

SpelEvaluator now iterates over the parameter map instead of using the Java 8 Stream API. Previously, expressions resulting in a null value failed in the collector as Java 8 streams require non-null values for map values.

Closes #2904

src/test/java/org/springframework/data/repository/query/SpelEvaluatorUnitTests.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.repository.query;
+
+import static org.assertj.core.api.Assertions.*;
+
+import java.lang.reflect.Method;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.data.repository.query.SpelQueryContext.SpelExtractor;
+
+/**
+ * Unit tests for {@link SpelEvaluator}.
+ *
+ * @author Mark Paluch
+ */
+class SpelEvaluatorUnitTests {
+
+	final SpelQueryContext context = SpelQueryContext.of((counter, s) -> String.format("__$synthetic$__%d", counter + 1),
+			String::concat);
+
+	@Test // GH-2904
+	void shouldEvaluateExpression() throws Exception {
+
+		SpelExtractor extractor = context.parse("SELECT :#{#value}");
+		Method method = MyRepository.class.getDeclaredMethod("simpleExpression", String.class);
+		SpelEvaluator evaluator = new SpelEvaluator(QueryMethodEvaluationContextProvider.DEFAULT,
+				new DefaultParameters(method), extractor);
+
+		assertThat(evaluator.getQueryString()).isEqualTo("SELECT :__$synthetic$__1");
+		assertThat(evaluator.evaluate(new Object[] { "hello" })).containsEntry("__$synthetic$__1", "hello");
+	}
+
+	@Test // GH-2904
+	void shouldAllowNullValues() throws Exception {
+
+		SpelExtractor extractor = context.parse("SELECT :#{#value}");
+		Method method = MyRepository.class.getDeclaredMethod("simpleExpression", String.class);
+		SpelEvaluator evaluator = new SpelEvaluator(QueryMethodEvaluationContextProvider.DEFAULT,
+				new DefaultParameters(method), extractor);
+
+		assertThat(evaluator.getQueryString()).isEqualTo("SELECT :__$synthetic$__1");
+		assertThat(evaluator.evaluate(new Object[] { null })).containsEntry("__$synthetic$__1", null);
+	}
+
+	interface MyRepository {
+
+		void simpleExpression(String value);
+
+	}
+}