Fix errors reported by psalm

Author: ADmad

src/DebugSql.php

@@ -76,6 +76,7 @@ public static function sql(Query $query, $showValues = true, $showHtml = null, $
             $sql = static::interpolate($sql, $query->getValueBinder()->bindings());
         }
 
+        /** @var array $trace */
         $trace = Debugger::trace(['start' => 1, 'depth' => $stackDepth + 2, 'format' => 'array']);
         $file = isset($trace[$stackDepth]) ? $trace[$stackDepth]['file'] : 'n/a';
         $line = isset($trace[$stackDepth]) ? $trace[$stackDepth]['line'] : 0;

src/DebugTimer.php

@@ -211,6 +211,6 @@ public static function requestStartTime()
             $startTime = env('REQUEST_TIME');
         }
 
-        return $startTime;
+        return (float)$startTime;
     }
 }

src/Panel/DeprecationsPanel.php

@@ -138,7 +138,7 @@ public function summary()
             $data = $this->_prepare();
         }
 
-        return array_reduce($data, function ($carry, $item) {
+        return (string)array_reduce($data, function ($carry, $item) {
             if (empty($item)) {
                 return $carry;
             }