Cannot filter between unset durations and configured zero-duration values

[me-and]

Zero durations match the empty string, and therefore there is no way to filter tasks that have explicit zero duration values.

The clearest example of the problem is thus:

$ task calc PT0S = '""'
true

I'd expect that result to be false.

The real problem for me is that I have a duration UDA and I'd like to be able to filter tasks where it's set, but I can't distinguish between cases where it's set to a zero value and where it's not set at all. Example output below, with extraneous lines removed:

$ task rc:taskrcdemo rc.data.location:taskdemo config uda.durationdemo.type duration

$ task rc:taskrcdemo rc.data.location:taskdemo add Demo task without duration
Created task 1.

$ task rc:taskrcdemo rc.data.location:taskdemo add Demo task with non-zero duration durationdemo:P1D
Created task 2.

$ task rc:taskrcdemo rc.data.location:taskdemo add Demo task with zero duration durationdemo:P0D
Created task 3.

$ task rc:taskrcdemo rc.data.location:taskdemo all
ID St UUID     Age   Description
 3 P  81709d81  5s   Demo task with zero duration
 2 P  716417ab 15s   Demo task with non-zero duration
 1 P  daa8f55b 27s   Demo task without duration

$ task rc:taskrcdemo rc.data.location:taskdemo 'durationdemo = ""' all  # I expect this to only show task 1, the task without a `durationdemo` value
ID St UUID     Age   Description
 3 P  81709d81 22s   Demo task with zero duration
 1 P  daa8f55b 44s   Demo task without duration

$ task rc:taskrcdemo rc.data.location:taskdemo 'durationdemo != ""' all  # I expect this to show tasks 2 and 3, the tasks with `durationdemo` values
ID St UUID     Age   Description
 2 P  716417ab 38s   Demo task with non-zero duration

Diag output below:

$ task rc:taskrcdemo rc.data.location:taskdemo diag
TASKRC override: taskrcdemo
TASKDATA override: taskdemo

task 2.6.2
   Platform: Linux

Compiler
    Version: 11.2.0
       Caps: +stdc +stdc_hosted +LP64 +c8 +i32 +l64 +vp64 +time_t64
 Compliance: C++17

Build Features
      CMake: 3.23.0
    libuuid: libuuid + uuid_unparse_lower
  libgnutls: 3.7.3
 Build type: None

Configuration
       File: taskrcdemo (found), 1402 bytes, mode 100644
       Data: taskdemo (found), dir, mode 40755
    Locking: Enabled
         GC: Enabled
    $VISUAL: vim
     Server:
         CA: -
Certificate: -
        Key: -
      Trust: strict
    Ciphers: NORMAL
      Creds:

Hooks
     System: Enabled
   Location: taskdemo/hooks
             (-none-)

Tests
   Terminal: 355x76
       Dups: Scanned 3 tasks for duplicate UUIDs:
             No duplicates found
 Broken ref: Scanned 3 tasks for broken references:
             No broken references found

Configuration override rc.data.location:taskdemo