checkstyle.xml: add a RegexpSinglelineJava module

Author: stephengold

config/checkstyle/checkstyle.xml

@@ -83,6 +83,12 @@
             <property name="tokens"
                       value="LITERAL_DO, LITERAL_ELSE, LITERAL_FOR, LITERAL_IF, LITERAL_WHILE"/>
         </module>
+        <module name="RegexpSinglelineJava">
+            <property name="format" value="\{[ ]+\}"/>
+            <property name="message" value="Empty blocks should have no spaces. Empty blocks
+                                   may only be represented as '{}' when not part of a
+                                   multi-block statement (4.1.3)"/>
+        </module>
         <module name="RightCurly">
             <property name="id" value="RightCurlyAlone"/>
             <property name="option" value="alone"/>