Fix #732 - Disable the select all button on Export tab if list is empty

src/org/opendatakit/briefcase/export/ExportForms.java

@@ -158,7 +158,11 @@ public boolean someSelected() {
   }
 
   public boolean allSelected() {
-    return forms.stream().allMatch(FormStatus::isSelected);
+    return !forms.isEmpty() && forms.stream().allMatch(FormStatus::isSelected);
+  }
+
+  public boolean isEmpty() {
+    return forms.isEmpty();
   }
 
   public boolean noneSelected() {

src/org/opendatakit/briefcase/ui/export/ExportPanel.java

@@ -149,6 +149,11 @@ private void updateSelectButtons() {
     } else {
       form.toggleSelectAll();
     }
+
+    if (forms.isEmpty())
+      form.disableSelectAll();
+    else
+      form.enableSelectAll();
   }
 
   public static ExportPanel from(BriefcasePreferences exportPreferences, BriefcasePreferences appPreferences, BriefcasePreferences pullPrefs, Analytics analytics, FormCache formCache, Http http) {

src/org/opendatakit/briefcase/ui/export/ExportPanelForm.java

@@ -111,6 +111,14 @@ void onChange(Runnable callback) {
     formsTable.onChange(callback);
   }
 
+  void disableSelectAll() {
+    selectAllButton.setEnabled(false);
+  }
+
+  void enableSelectAll() {
+    selectAllButton.setEnabled(true);
+  }
+
   void toggleClearAll() {
     selectAllButton.setVisible(false);
     clearAllButton.setVisible(true);