Avoid NPE when replacement property does not exist

See gh-15394

spring-boot-project/spring-boot-properties-migrator/src/main/java/org/springframework/boot/context/properties/migrator/PropertiesMigrationReporter.java

@@ -137,6 +137,9 @@ class PropertiesMigrationReporter {
 		if (lastDot != -1) {
 			ConfigurationMetadataProperty property = this.allProperties
 					.get(fullId.substring(0, lastDot));
+			if (property == null) {
+				return null;
+			}
 			String type = property.getType();
 			if (type != null && type.startsWith(Map.class.getName())) {
 				int lastComma = type.lastIndexOf(',');

spring-boot-project/spring-boot-properties-migrator/src/test/java/org/springframework/boot/context/properties/migrator/PropertiesMigrationReporterTests.java

@@ -152,6 +152,19 @@ public class PropertiesMigrationReporterTests {
 						+ "'test.inconvertible' uses an incompatible target type");
 	}
 
+	@Test
+	public void invalidReplacementHandled() throws IOException {
+		this.environment.getPropertySources().addFirst(loadPropertySource("first",
+				"config/config-error-invalid-replacement.properties"));
+		String report = createErrorReport(
+				loadRepository("metadata/sample-metadata-invalid-replacement.json"));
+		assertThat(report).isNotNull();
+		assertThat(report).containsSubsequence("Property source 'first'",
+				"deprecated.six.test", "Line: 1", "Reason",
+				"Replacement key 'does.not.exist' uses an incompatible target type");
+		assertThat(report).doesNotContain("null");
+	}
+
 	private List<String> mapToNames(PropertySources sources) {
 		List<String> names = new ArrayList<>();
 		for (PropertySource<?> source : sources) {

spring-boot-project/spring-boot-properties-migrator/src/test/resources/config/config-error-invalid-replacement.properties

+deprecated.six.test=abc

spring-boot-project/spring-boot-properties-migrator/src/test/resources/metadata/sample-metadata-invalid-replacement.json

+{
+  "properties": [
+    {
+      "name": "deprecated.six.test",
+      "type": "java.lang.String",
+      "deprecation": {
+        "replacement": "does.not.exist",
+        "level": "error"
+      }
+    }
+  ]
+}
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