Disallow null offset in lead/lag window functions

[martint]

Per the SQL spec, the offset cannot be null.

<lead or lag function> ::=
  <lead or lag> <left paren> <lead or lag extent>
      [ <comma> <offset> [ <comma> <default expression> ] ] <right paren>
      [ <window function null treatment> ]

<offset> ::=
  <unsigned integer>

Moreover, it should be a constant value, but this is not being addressed in this change.

Fixes #19003

Release notes

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# General
* Disallow a null offset in {func}`lead` and {func}`lag`. ({issue}`19003`)