classlib: add CharSequence.isEmpty support (#683)

classlib/src/main/java/org/teavm/classlib/java/lang/TAbstractStringBuilder.java

@@ -559,6 +559,11 @@ class TAbstractStringBuilder implements TSerializable, TCharSequence {
         return buffer[index];
     }
 
+    @Override
+    public boolean isEmpty() {
+        return length == 0;
+    }
+
     protected TAbstractStringBuilder append(TCharSequence s, int start, int end) {
         return insert(length, s, start, end);
     }

classlib/src/main/java/org/teavm/classlib/java/lang/TCharSequence.java

@@ -20,6 +20,8 @@ public interface TCharSequence {
 
     char charAt(int index);
 
+    boolean isEmpty();
+
     TCharSequence subSequence(int start, int end);
 
     @Override

classlib/src/main/java/org/teavm/classlib/java/lang/TString.java

@@ -151,6 +151,7 @@ public class TString extends TObject implements TSerializable, TComparable<TStri
         return characters.length;
     }
 
+    @Override
     public boolean isEmpty() {
         return characters.length == 0;
     }