Allow StringSequence.isEmpty() to be compatible with JDK 15

JDK 15 introduces isEmpty() on CharSequence which clashes with the one
declared in StringSequence because it is not public.

See gh-21713

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

@@ -46,4 +46,5 @@
 	<suppress files="[\\/]src[\\/]intTest[\\/]java[\\/]" checks="SpringJavadoc" message="\@since" />
 	<suppress files="LinuxDomainSocket" checks="FinalClass" message="SockaddrUn" />
 	<suppress files="BsdDomainSocket" checks="FinalClass" message="SockaddrUn" />
+	<suppress files="StringSequence" checks="SpringMethodVisibility" />
 </suppressions>