Give get_effects(::MultiCallInfo) an identity for empty callinfos

[jishnub]

get_effects(mci::MultiCallInfo) is mapreduce(get_effects, merge_effects, mci.callinfos) with no init, so an empty MultiCallInfo throws instead of yielding effects. select_callsite already anticipates the empty case with its own error branch, but any effects display reaches get_effects before that.

EFFECTS_TOTAL is the identity of merge_effects (checked: merge_effects(EFFECTS_TOTAL, x) == x == merge_effects(x, EFFECTS_TOTAL) for EFFECTS_UNKNOWN and a partially-tainted Effects), so it is the right init.

Test: an empty MultiCallInfo now returns EFFECTS_TOTAL.

🤖 Generated with Claude Code

https://claude.ai/code/session_01NiXhCrQf5AJ2DmTmZ3FsSP

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (2c33b9a) to head (96c4b2a).
⚠️ Report is 101 commits behind head on master.

Files with missing lines	Patch %	Lines
src/compiler/callsite.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #708    +/-   ##
=======================================
  Coverage    0.00%   0.00%            
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  Files           9      22    +13     
  Lines        1556    1808   +252     
=======================================
- Misses       1556    1808   +252     

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