Fix MockHttpServletRequest InputStream for empty body

Prior to this commit, a recent change applied in gh-29125 changed the
behavior of `MockHttpServletRequest` instances. In case of an empty
request body, the returned `InputStream` would be static and could not
be reused across requests.
This could result in `java.io.IOException: Stream closed` exceptions if
a previous request was read.

This commit ensures that a new instance of an empty stream is returned
for each request instance.

Fixes gh-29901

spring-test/src/test/java/org/springframework/mock/web/MockHttpServletRequestTests.java

@@ -82,6 +82,17 @@ class MockHttpServletRequestTests {
 		assertThat(StreamUtils.copyToString(request.getInputStream(), Charset.defaultCharset())).isEqualTo("body");
 	}
 
+	@Test
+	void readEmptyInputStreamWorksAcrossRequests() throws IOException {
+		MockHttpServletRequest firstRequest = new MockHttpServletRequest();
+		firstRequest.getInputStream().readAllBytes();
+		firstRequest.getInputStream().close();
+
+		MockHttpServletRequest secondRequest = new MockHttpServletRequest();
+		secondRequest.getInputStream().readAllBytes();
+		secondRequest.getInputStream().close();
+	}
+
 	@Test
 	void setContentAndGetReader() throws IOException {
 		byte[] bytes = "body".getBytes(Charset.defaultCharset());