Make ramble consider '' as unset and error correctly

[rfbgo]

Setting n_ranks: '' (which is sometimes down by workspace manage experiments) used to give this misleading error:

  File "/Users/robertbird/ramble/var/ramble/repos/builtin/base_classes/application-base/base_class.py", line 1298, in run_phase
    phase_func(workspace, app_inst=self)
    ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/robertbird/ramble/var/ramble/repos/builtin/base_applications/hpl/base_application.py", line 363, in _calculate_values
    nNodes = int(expander.expand_var_name("n_nodes"))
ValueError: invalid literal for int() with base 10: ''
Processing phase calculate_values (2/7):  29%|===========>

This PR means that value is no ignored so the appropriate instruction to the user is issued requiring a valid value:

==> Error: Invalid number of required variables defined.
Two or more of the following are required to be defined.
  - n_ranks
  - processes_per_node
  - n_nodes
Experiment hpl.standard.generated only has:
  - processes_per_node
  - ```

[gemini-code-assist]

Code Review

This pull request refactors the MPI variable definition logic in base_class.py to prevent empty or None variables from being incorrectly marked as defined, and corrects the logic for tracking missing MPI variables. The review feedback suggests enhancing the validation of MPI variables by stripping whitespace to robustly handle whitespace-only strings.

[ramble-pr-bot]

Ramble Performance Test Metrics

Results produced with commit: 2b7218e

Test Name	Outcome	Duration (s)	Most Recent Run (s)	Last 5 Avg (s)
test_analyze_large_file	passed	2.0713	2.1005 (de4ccce)	2.0984
test_large_template_expansion	passed	1.6883	1.6327 (de4ccce)	1.6532
test_many_experiments	passed	25.2698	24.6595 (de4ccce)	25.4157
test_many_objects_defaults	passed	14.5563	14.4773 (de4ccce)	14.4945
test_matrix_filter_perf	passed	1.1682	1.1632 (de4ccce)	1.1704

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.48%. Comparing base (de4ccce) to head (2b7218e).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1713   +/-   ##
========================================
  Coverage    93.48%   93.48%           
========================================
  Files          367      367           
  Lines        35916    35918    +2     
========================================
+ Hits         33575    33577    +2     
  Misses        2341     2341           

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