renameColumn will changed default from NULL to 'NULL'

[kingshark8848]

DB version: MariaDB 10.2.7
Laravel version: 5.5
doctrine/dbal version 2.6

issue description:
When use code below to change column name,

Schema::table('my_table', function (Blueprint $table) {
    $table->renameColumn('from', 'to');
});

it will cause error:
"SQLSTATE[22007]: Invalid datetime format: 1292 Incorrect datetime value: 'NULL' for column 'c__x5a' at row 1 (SQL: ALTER TABLE xxx CHANGE c__x5 c__x5a DATETIME DEFAULT 'NULL')"

the column's default value is NULL, but was changed to 'NULL' (with single quotes), which caused the error.

[belgattitude]

fixed in master, patch will be released in 2.7

[Ocramius]

See #2824 and #2825. Closing as duplicate.

[github-actions]

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