fix(appset): avoid panic when no steps in rollingSync

[CefBoud]

Closes #18817.

Description

When using Applicationset's rolling sync, having no steps in the manifest results in a panic. This PR attempts to fix that by ensuring a positive number of steps.

Additionally, applications not selected by MatchExpressions are still treated as part of Step 1 (actually Step 0 due to 1-based indexing) because the appStepMap defaults to 0 and there is no validation for the presence in the map.

These applications are not synced by the rolling sync. I suggest assigning them a value of -1 to clearly indicate that they are excluded from the sync and must be manually synced.
Checklist:

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.01%. Comparing base (9b11b21) to head (e02882c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #20357      +/-   ##
==========================================
+ Coverage   55.99%   56.01%   +0.01%     
==========================================
  Files         322      322              
  Lines       44759    44761       +2     
==========================================
+ Hits        25062    25072      +10     
+ Misses      17093    17089       -4     
+ Partials     2604     2600       -4     

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[todaywasawesome]

LGTM