Add Checkstyle's SingleSpaceSeparator rule (#7686)

Author: eddumelendez

config/checkstyle/checkstyle.xml

@@ -58,6 +58,7 @@
         </module>
         <module name="OneStatementPerLineCheck"/>
         <module name="ModifierOrder" />
+        <module name="SingleSpaceSeparator" />
     </module>
     <module name="RegexpSingleline">
         <property name="format" value="^\s*\*\s*@author"/>