Separate functions for allowing/disallowing empty csv files

[tsmbland]

Description

read_csv was originally intended to disallow empty csv files, but this was lost somewhere along the way.

That said, there are some instances where we might want to allow empty csv files, such as agent_search_space if the user doesn't want to apply any search spaces.

I've created read_csv_optional to allow empty csv files, modified read_csv to disallow empty csv files, and put the shared code in _read_csv_internal.

So far only agent_search_space is using read_csv_optional. There are some other empty csv files (process_flow_share_constraints etc.) but I believe these aren't read in yet

Fixes #106

Type of change

• Bug fix (non-breaking change to fix an issue)
• New feature (non-breaking change to add functionality)
• Refactoring (non-breaking, non-functional change to improve maintainability)
• Optimization (non-breaking change to speed up the code)
• Breaking change (whatever its nature)
• Documentation (improve or add documentation)

Key checklist

• All tests pass: $ cargo test
• The documentation builds and looks OK: $ cargo doc

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 3 lines in your changes missing coverage. Please review.

Project coverage is 95.11%. Comparing base (cd7ea10) to head (7d7372f).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/input.rs	90.00%	0 Missing and 2 partials ⚠️
src/input/agent/search_space.rs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #463      +/-   ##
==========================================
- Coverage   95.13%   95.11%   -0.03%     
==========================================
  Files          32       32              
  Lines        4732     4751      +19     
  Branches     4732     4751      +19     
==========================================
+ Hits         4502     4519      +17     
  Misses        119      119              
- Partials      111      113       +2     

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[alexdewar]

I think it would be a bit cleaner to have _read_csv_internal return the Vec, then have the caller check whether it's empty or not. This is also ok though.