[SDK-3328] Fix classCastException from while reading PE variable

[shivamsharma2710]

Issue Summary: When a PE variable of Long type is defined with a null default value, and the dashboard overrides it with a Double value, The SDK throws a ClassCastException (Double cannot be cast to Long).

Fix: Restricted the null usage as default value for PE variables.

[Anush-Shand]

Unit tests are failing @shivamsharma2710