Treat nil from any SimpleCov session as non-executable

SimpleCov's track_files placeholder uses a parser-based heuristic to
mark executable lines in files no process loaded. Ruby's Coverage
module is authoritative when the file was loaded. The previous merge
combined nil and 0 into 0, which inflated total by counting heuristic
"executable" lines that the real Coverage run had marked non-executable.
Switch to nil-wins so the authoritative classification overrides the
heuristic.

Author: comandeo-mongo

.evergreen/lib/coverage_gate.rb

@@ -84,11 +84,15 @@ def parse_resultset(data)
     end
   end
 
+  # SimpleCov's track_files placeholder uses a heuristic to mark executable
+  # lines in files no process actually loaded. Ruby's Coverage module is
+  # authoritative when the file was loaded, so a nil from any session wins
+  # over a heuristic 0 from another.
   def merge_lines(a, b)
     a.zip(b).map do |x, y|
-      next nil if x.nil? && y.nil?
+      next nil if x.nil? || y.nil?
 
-      (x.is_a?(Integer) ? x : 0) + (y.is_a?(Integer) ? y : 0)
+      x + y
     end
   end
 

spec/mongo/coverage_gate_spec.rb

@@ -135,6 +135,21 @@ def write_baseline(files)
         write_baseline('lib/mongo/foo.rb' => { 'covered' => 3, 'total' => 3 })
         expect(gate.check).to eq(0)
       end
+
+      it 'treats a nil from any session as non-executable' do
+        # Session A loaded the file: Ruby Coverage marked lines 0, 4, 5 as
+        # non-executable (nil). Session B used the track_files heuristic and
+        # tagged those same lines as executable-not-hit (0). The merge must
+        # not inflate total by counting heuristic 0s where a real run said nil.
+        write_multi_session_resultset(
+          [
+            { 'lib/mongo/foo.rb' => [ nil, 1, 1, 0, nil, nil ] },
+            { 'lib/mongo/foo.rb' => [ 0,   0, 0, 0, 0,   0   ] },
+          ]
+        )
+        write_baseline('lib/mongo/foo.rb' => { 'covered' => 2, 'total' => 3 })
+        expect(gate.check).to eq(0)
+      end
     end
   end