Handle blank Accept-Language header in AcceptHeaderLocaleResolver

When the Accept-Language header is present but blank, defaultLocale
was ignored and fell back to Locale.getDefault() (JVM system locale).
A blank header represents the same intent as an absent header.

Closes gh-36513

Signed-off-by: elgun.shukurov <elgun.sukurov@kapitalbank.az>

spring-webmvc/src/main/java/org/springframework/web/servlet/i18n/AcceptHeaderLocaleResolver.java

@@ -85,7 +85,7 @@ public class AcceptHeaderLocaleResolver extends AbstractLocaleResolver {
 	@Override
 	public Locale resolveLocale(HttpServletRequest request) {
 		Locale defaultLocale = getDefaultLocale();
-		if (defaultLocale != null && request.getHeader("Accept-Language") == null) {
+		if (defaultLocale != null && !StringUtils.hasText(request.getHeader("Accept-Language"))) {
 			return defaultLocale;
 		}
 		Locale requestLocale = request.getLocale();

spring-webmvc/src/test/java/org/springframework/web/servlet/i18n/AcceptHeaderLocaleResolverTests.java

@@ -105,6 +105,14 @@ class AcceptHeaderLocaleResolverTests {
 		assertThat(this.resolver.resolveLocale(request)).isEqualTo(US);
 	}
 
+	@Test
+	void defaultLocaleWithBlankAcceptLanguageHeader() {
+		this.resolver.setDefaultLocale(JAPANESE);
+		MockHttpServletRequest request = new MockHttpServletRequest();
+		request.addHeader("Accept-Language", "");
+		assertThat(this.resolver.resolveLocale(request)).isEqualTo(JAPANESE);
+	}
+
 
 	private HttpServletRequest request(Locale... locales) {
 		MockHttpServletRequest request = new MockHttpServletRequest();