Cannot filter between unset durations and configured zero-duration values

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
```