Support empty OVER() clause in Window Specification

[tunetheweb]

Fixes #1293

[codecov]

Codecov Report

Merging #1294 (59f0f86) into main (00fca3a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1294   +/-   ##
=======================================
  Coverage   96.13%   96.13%           
=======================================
  Files         124      124           
  Lines        8609     8609           
=======================================
  Hits         8276     8276           
  Misses        333      333           

Flag	Coverage Δ
dbt018-py38	60.68% <ø> (ø)
dbt019-py38	60.68% <ø> (ø)
dbt020-py38	60.64% <ø> (ø)
py36	94.86% <ø> (ø)
py37	94.89% <ø> (ø)
py38	94.88% <ø> (ø)
py39	94.88% <ø> (ø)

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Impacted Files	Coverage Δ
src/sqlfluff/dialects/dialect_ansi.py	99.79% <ø> (ø)

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[barrywhart]

Nice work!