PCP: do not set exe to empty string

In case the executable is an empty string, e.g. if pcp is run by an
unprivileged user, do not set procExe to an empty value, which breaks
the formatting of the PROCEXE column and the merged-cmdline logic.

pcp/PCPProcessList.c

@@ -317,7 +317,7 @@ static void PCPProcessList_updateCmdline(Process* process, int pid, int offset,
    Process_updateComm(process, comm);
 
    if (PCPMetric_instance(PCP_PROC_EXE, pid, offset, &value, PM_TYPE_STRING)) {
-      Process_updateExe(process, value.cp);
+      Process_updateExe(process, value.cp[0] ? value.cp : NULL);
       free(value.cp);
    }
 }