fix: OCI8 Forge::modifyColumn() changes behavior to NULL by default

For consistency.

Author: kenjis

system/Database/OCI8/Forge.php

@@ -120,10 +120,17 @@ protected function _alterTable(string $alterType, string $table, $field)
                 // If a null constraint is added to a column with a null constraint,
                 // ORA-01451 will occur,
                 // so add null constraint is used only when it is different from the current null constraint.
-                $isWantToAddNull    = strpos($field[$i]['null'], ' NOT') === false;
-                $currentNullAddable = $nullableMap[$field[$i]['name']];
+                // If a not null constraint is added to a column with a not null constraint,
+                // ORA-01442 will occur.
+                $wantToAddNull   = strpos($field[$i]['null'], ' NOT') === false;
+                $currentNullable = $nullableMap[$field[$i]['name']];
 
-                if ($isWantToAddNull === $currentNullAddable) {
+                if ($wantToAddNull === true && $currentNullable === true) {
+                    $field[$i]['null'] = '';
+                } elseif ($field[$i]['null'] === '' && $currentNullable === false) {
+                    // Nullable by default
+                    $field[$i]['null'] = ' NULL';
+                } elseif ($wantToAddNull === false && $currentNullable === false) {
                     $field[$i]['null'] = '';
                 }
             }