[FR] Remove blank :effort:s from taskjuggler export

[Daniel Nemenyi]

Dear wonderful Orgers,

Something I think would make the taskjuggler export even smoother would be if 
empty :effort:'s were simply skipped over, rather than exported to the tjp 
without a value and triggering a tj3 error. Let me explain.

Say we had the following org file, with one big feature:

---

#+COLUMNS: %70ITEM(Task) %Effort %allocate

* Big feature                                          :taskjuggler_project:
  :PROPERTIES:
  :EFFORT:   4d
  :ALLOCATE: alice, john
  :END:
* Resources                                            :taskjuggler_resource:
** Alice P Hacker
** John McCarthy

---

I decide that I want to break up that big task into subtasks, so I add them and 
switch into column view to add the efforts and allocations. Crucially, also in 
column view, I delete the effort estimate (using e) from the Big Feature, as 
otherwise taskjuggler would error that Big feature (a parent node) should not 
have an effort estimate when it has children.

This typical behaviour (for me) leaves me with the following.

---

#+COLUMNS: %70ITEM(Task) %Effort %allocate

* Big feature                                          :taskjuggler_project:
  :PROPERTIES:
  :EFFORT:           # Note empty :effort:
  :END:
** Design feature
   :PROPERTIES:
   :EFFORT:   1d
   :ALLOCATE: alice
   :END:
** Code feature
   :PROPERTIES:
   :EFFORT:   2d
   :ALLOCATE: john
   :END:
** Test feature
   :PROPERTIES:
   :EFFORT:   1d
   :ALLOCATE: alice
   :END:
* Resources                                            :taskjuggler_resource:
** Alice P Hacker
** John McCarthy

---

Alas the empty :effort: node causes tj3 to fail with a cryptic 'Error: 
Unexpected token 'task' found. Expecting one of :INTEGER, :FLOAT', as the 
effort key is exported to the tjp file without a value:

---

task big "Big feature" {
  effort 
  task design "Design feature" {
...

---

My solution would be for org-taskjuggler-export to simply skip over empty 
effort symbols. Another solution might be for column view to remove symbols 
that have been deleted - however this might have unintended consequences?

Thanks for taking this into account, and for the amazing tool that is org mode!
Daniel