BUG: fix for empty custom commands

Source/cmLocalUnixMakefileGenerator3.cxx

@@ -2527,6 +2527,8 @@ cmLocalUnixMakefileGenerator3
     // Build the command line in a single string.
     const cmCustomCommandLine& commandLine = *cl;
     std::string cmd = commandLine[0];
+    if (cmd.size())
+      {
       cmSystemTools::ReplaceString(cmd, "/./", "/");
       cmd = this->Convert(cmd.c_str(),START_OUTPUT);
       if(cmd.find("/") == cmd.npos &&
@@ -2544,6 +2546,7 @@ cmLocalUnixMakefileGenerator3
       
       commands1.push_back(cmd);
       }
+    }
   
   // stick this group of commands into a cd of the proper path
   // Build the jump-and-build command list.