fix: do not try to write empty ftof_ctof_vtdiff timeline (#313)

Author: c-dilks

src/main/java/org/jlab/clas/timeline/analysis/alert/alert_atof_tdc.groovy

@@ -41,7 +41,7 @@ def has_data = new AtomicBoolean(false)
   def write() {
 
     if(!has_data.get()) {
-      System.err.println "ERROR: no data for this ALERT timeline, not producing"
+      System.err.println "WARNING: no data for this timeline, not producing"
       return
     }
 

src/main/java/org/jlab/clas/timeline/analysis/ftof/ftof_ctof_vtdiff.groovy

@@ -1,19 +1,22 @@
 package org.jlab.clas.timeline.analysis
 import java.util.concurrent.ConcurrentHashMap
+import java.util.concurrent.atomic.AtomicBoolean
 import org.jlab.groot.data.TDirectory
 import org.jlab.groot.data.GraphErrors
 import org.jlab.clas.timeline.fitter.MoreFitter
 
 class ftof_ctof_vtdiff {
 
 def data = new ConcurrentHashMap()
+def has_data = new AtomicBoolean(false)
 
 def processRun(dir, run) {
   (1..6).collect{sec->
     def h1 = dir.getObject("/tof/ftof-ctof_vtdiff_S${sec}")
     if(h1.integral() > 0.0) {
       def f1 = MoreFitter.fitgaus(h1)
       data.computeIfAbsent(sec, {[]}).add([run:run, h1:h1, f1:f1, mean:f1.getParameter(1), sigma:f1.getParameter(2).abs()])
+      has_data.set(true)
     }
   }
 }
@@ -22,6 +25,11 @@ def processRun(dir, run) {
 
 def write() {
 
+  if(!has_data.get()) {
+    System.err.println "WARNING: no data for this timeline, not producing"
+    return
+  }
+
   ['mean', 'sigma'].each{ name ->
     TDirectory out = new TDirectory()
     out.mkdir('/timelines')