Fix a warning when DateTime column has empty value

[xinhus]

Dear reviewer,

When you have a DateTime column and the value is empty a warning was raised.

The code first checks the string at position 0 then check if is "empty".

On PHP version <= 7.4 A warning is raised if you try to access [0] of an "Uninitialized" string and $foo = '' is an "Uninitialized" string 🤷‍♂️ .

(I haven't check if this behavior still exists on PHP >=8.0).

The change is just a small change in the order of an existing code.

[muglug]

Thanks!