Clarify what a string being empty means

Should close https://github.com/godotengine/godot-docs/issues/2432

doc/classes/String.xml

@@ -381,7 +381,7 @@
 			<return type="bool">
 			</return>
 			<description>
-				Returns [code]true[/code] if the string is empty.
+				Returns [code]true[/code] if the length of the string equals [code]0[/code].
 			</description>
 		</method>
 		<method name="ends_with">