Negative tests for array length encoding

tests/test_proof/bad/array_length-empty.catala_en

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+## Test
+
+```catala
+declaration scope A:
+  context x content collection integer
+  context y content boolean
+
+scope A:
+  definition x equals [0; 5]
+  definition y under condition (number of x) > 0 consequence equals true
+```

tests/test_proof/bad/array_length-overlap.catala_en

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+## Test
+
+```catala
+declaration scope A:
+  context x content collection integer
+  context y content boolean
+
+scope A:
+  definition x equals [0; 5]
+  definition y under condition (number of x) >= 0 consequence equals true
+  definition y under condition (number of x) = 1 consequence equals false
+```

tests/test_proof/bad/output/array_length-empty.catala_en.Proof

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+[ERROR] [A.y] This variable might return an empty error:
+ --> tests/test_proof/bad/array_length-empty.catala_en
+  | 
+6 |   context y content boolean
+  |           ^
+  + Test
+Counterexample generation is disabled so none was generated.
+[RESULT] [A.y] No two exceptions to ever overlap for this variable
+[RESULT] [A.x] This variable never returns an empty error
+[RESULT] [A.x] No two exceptions to ever overlap for this variable

tests/test_proof/bad/output/array_length-overlap.catala_en.Proof

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+[RESULT] [A.y] This variable never returns an empty error
+[ERROR] [A.y] At least two exceptions overlap for this variable:
+ --> tests/test_proof/bad/array_length-overlap.catala_en
+  | 
+6 |   context y content boolean
+  |           ^
+  + Test
+Counterexample generation is disabled so none was generated.
+[RESULT] [A.x] This variable never returns an empty error
+[RESULT] [A.x] No two exceptions to ever overlap for this variable