Fixed: opening Execution History (from the modern Test Case Viewer gui/templates/testcases/tcView.html → toolbar button → legacy lib/execute/execHistory.php) and running Quick Search on a test project without test cases wrote two error clusters into the Event Viewer.

1. SQL error (source: DATABASE), event log showed:

   1064 - You have an error in your SQL syntax ... near ')' at line 1
   SELECT COUNT(DISTINCT(NH_TC.id)) FROM nodes_hierarchy NH_TC
     JOIN nodes_hierarchy NH_TCV ON NH_TCV.parent_id = NH_TC.id
     JOIN tcversions TCV ON NH_TCV.id = TCV.id ...
   WHERE 1=1 AND NH_TCV.parent_id IN ()
   

2. E_WARNING (source: GUI):

   Array to string conversion - gui/templates_c/...execHistory.tpl.php - Line 230
   

   The page still rendered, but printed a literal Array in the custom-field cell of every execution row and polluted the Event Viewer.

The issue title attributed this to execHistory.php, but the query shape (COUNT(DISTINCT(NH_TC.id)), aliases NH_TC/NH_TCV) belongs to the test case search code:

• lib/testcases/tcSearch.php (legacy Quick Search)
• api/search/index.php (modernized Quick Search BFF)

Both build their testcase-id filter like this:

$a_tcid = null;
$tproject_mgr->get_all_testcases_id($tprojectID, $a_tcid);
if (!is_null($a_tcid)) {
    $filter['by_tc_id'] = " AND NH_TCV.parent_id IN (" . implode(",", $a_tcid) . ") ";
}

On MariaDB, testproject::get_all_testcases_id() uses the CTE branch which always assigns $tcIDs = $itemSet — an empty array [] (never null) when the project has no test cases. The is_null() guard was therefore dead code for that case and the code assembled IN () → MySQL error 1064.

Reproduced pre-fix: Quick Search on a case-less project logged exactly the 1064 above from both call sites.

getCustomFields() in lib/execute/execHistory.php did:

$dummy = (array)$tcaseMgr->html_table_of_custom_field_values(...);
$cf[$exec_id] = (count($dummy) > 0) ? $dummy : '';

html_table_of_custom_field_values() returns an HTML string ($cf_smarty). Casting it to (array) produced [0 => '<table>…'] (or [0 => ''] when no custom fields exist — count is still 1), so $gui->cfexec[$exec_id] was always an array. The template echoes it directly:

{assign var="cf_value_info" value=$gui->cfexec[$tcv_exec.execution_id]}
{$cf_value_info}

→ PHP 8.x raises Array to string conversion at compiled line 230 and prints the literal word "Array".

Minimal guards at the two defective spots; no contract changes to shared library code:

Alternatives rejected:

• Fixing get_all_testcases_id() to return null for empty sets — changes the function contract; many other callers (searchCommands, uncoveredTestCases, planImport, REST v1/v2/v3) rely on the array return.
• Iterating $cf_value_info in the templates — would need changes in two templates and still leave the data producer lying about its type; fixing the producer fixes every consumer.
• !empty($a_tcid) — equivalent but less explicit; kept the null check to mirror surrounding style.

The isset() guard on grants->exec_edit_notes also hardens a PHP 8.x edge: an execution row belonging to a test plan the user cannot access (e.g. session project switched after the execution was recorded) previously warned with Trying to access array offset on null / Undefined array key.

Regression suite 36 in tmp/TLU_Test_Cases.md — 10/10 PASS:

• Pre-fix repro confirmed both clusters (events #10, #15, #16 on the fixture instance).
• Post-fix: exec history renders rows cleanly (no warnings, no literal "Array"); search on empty project returns the clean "empty testproject" state via both legacy screen and BFF JSON; search on a project with cases still finds them; toolbar flow tcView → Execution History verified in the browser; Event Viewer shows zero new Error/Warning entries.

• lib/testcases/tcSearch.php
• api/search/index.php
• lib/execute/execHistory.php
• gui/templates/dashio/execute/execHistory.tpl
• gui/templates/tl-classic/execute/execHistory.tpl