[8.x] Fix null values for cursor pagination

src/Illuminate/Database/Concerns/BuildsQueries.php

@@ -315,11 +315,18 @@ protected function paginateUsingCursor($perPage, $columns = ['*'], $cursorName =
                 $builder->where(function (self $builder) use ($addCursorConditions, $cursor, $orders, $i) {
                     ['column' => $column, 'direction' => $direction] = $orders[$i];
 
-                    $builder->where(
-                        $this->getOriginalColumnNameForCursorPagination($this, $column),
-                        $direction === 'asc' ? '>' : '<',
-                        $cursor->parameter($column)
-                    );
+                    $param = $cursor->parameter($column);
+                    $originalColumn = $this->getOriginalColumnNameForCursorPagination($this, $column);
+
+                    if (is_null($param)) {
+                        $this->whereClauseForCursorNullValue($originalColumn, $builder, $direction);
+                    } else {
+                        $builder->where(
+                            $originalColumn,
+                            $direction === 'asc' ? '>' : '<',
+                            $param
+                        );
+                    }
 
                     if ($i < $orders->count() - 1) {
                         $builder->orWhere(function (self $builder) use ($addCursorConditions, $column, $i) {
@@ -341,6 +348,27 @@ protected function paginateUsingCursor($perPage, $columns = ['*'], $cursorName =
         ]);
     }
 
+    /**
+     * Handle creating a where clause for when a cursor value is null.
+     *
+     * @param  string  $column
+     * @param  \Illuminate\Database\Query\Builder|\Illuminate\Database\Eloquent\Builder  $builder
+     * @param  string  $direction
+     * @return void
+     */
+    protected function whereClauseForCursorNullValue($column, $builder, $direction)
+    {
+        $ascending = $direction === 'asc';
+        if (! $builder->getGrammar()->nullsAreOrderedFirst()) {
+            $ascending = ! $ascending;
+        }
+        if ($ascending) {
+            $builder->whereNotNull($column);
+        } else {
+            $builder->whereNull($column);
+        }
+    }
+
     /**
      * Get the original column name of the given column, without any aliasing.
      *

src/Illuminate/Database/Query/Grammars/Grammar.php

@@ -1284,4 +1284,14 @@ public function getOperators()
     {
         return $this->operators;
     }
+
+    /**
+     * Nulls are ordered first when ordring in ascending order.
+     *
+     * @return bool
+     */
+    public function nullsAreOrderedFirst()
+    {
+        return true;
+    }
 }

src/Illuminate/Database/Query/Grammars/PostgresGrammar.php

@@ -409,4 +409,14 @@ protected function wrapJsonPathAttributes($path)
                         : "'$attribute'";
         }, $path);
     }
+
+    /**
+     * Nulls are ordered first when ordring in ascending order.
+     *
+     * @return bool
+     */
+    public function nullsAreOrderedFirst()
+    {
+        return false;
+    }
 }