I was able to solve this problem by only adding $stmt->bindValue for fields that were not empty by adding this:
foreach ($searchTerms as $field => $value) {
if ($value != null && $value != '' && $value != ' ' ) {
// Based on the key name in the array, decide which SQL statement to add to the array to be constructed
switch ($field) {
case 'locnum':
$stmt->bindValue(':locnum', $locnum, PDO::PARAM_INT);
break;
case 'drawnum':
$stmt->bindValue(':drawnum', $drawnum, PDO::PARAM_INT);
break;
case 'drawdate':
$stmt->bindValue(':drawdate', $drawdate, PDO::PARAM_STR);
break;
}
}
}