fix(ci): skip files without modified functions (ethereum-optimism#11891)

Updates test-heavy-fuzz-modified-tests to skip any files that
don't have any modified fuzz tests.

packages/contracts-bedrock/scripts/testing/test-heavy-fuzz-modified-tests.sh

@@ -88,6 +88,11 @@ for FILE in $CHANGED_FILES; do
     # Extract function names and their line numbers from the entire file
     FUNCTION_LINES=$(awk '/function testFuzz_|function invariant_/ {print FNR, $0}' "$FILE")
 
+    # If there are no function lines, skip the file.
+    if [ -z "$FUNCTION_LINES" ]; then
+        continue
+    fi
+
     # Reverse the function lines so we can match the last function modified.
     # We'd otherwise end up matching the first function with a line number less
     # than the modified line number which is not what we want.