Make StringSequence.isEmpty() public

Make `StringSequence.isEmpty()` public for compatibility with JDK 15.

Closes gh-23440

spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jar/StringSequence.java

@@ -72,7 +72,11 @@ final class StringSequence implements CharSequence {
 		return new StringSequence(this.source, subSequenceStart, subSequenceEnd);
 	}
 
-	boolean isEmpty() {
+	/**
+	 * Returns {@code true} if the sequence is empty. Public to be compatible with JDK 15.
+	 * @return {@code true} if {@link #length()} is {@code 0}, otherwise {@code false}
+	 */
+	public boolean isEmpty() {
 		return length() == 0;
 	}
 

src/checkstyle/checkstyle-suppressions.xml

@@ -42,4 +42,5 @@
 	<suppress files="SampleJUnitVintageApplicationTests" checks="SpringJUnit5" />
 	<suppress files="[\\/]spring-boot-docs[\\/]" checks="SpringJavadoc" message="\@since" />
 	<suppress files="[\\/]spring-boot-smoke-tests[\\/]" checks="SpringJavadoc" message="\@since" />
+	<suppress files="StringSequence" checks="SpringMethodVisibility"/>
 </suppressions>