Server checks for ticket prices are missing

[mrsaicharan1]

Describe the bug
Server checks are required for ticket prices. This would also allow decimal values greater than zero to pass through [similar to eventbrite]

Additional context
Corresponding FE PR which requires these checks : fossasia/open-event-frontend#3341

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[ashutosh1919]

@mrsaicharan1 Please see my closed branch which passes all the integration tests and change accordingly.