vega: Fix custom templates when data is not the first dict in the t…

[daavoo]

…emplate.

The has_anchor check was returning at the first dictionary encounter so even if the template had a valid data anchor, it would fail if it was not the first dictionary.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.85 🎉

Comparison is base (a4099d3) 95.52% compared to head (ef1a925) 96.37%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #132      +/-   ##
==========================================
+ Coverage   95.52%   96.37%   +0.85%     
==========================================
  Files          19       19              
  Lines         759      772      +13     
  Branches      120      122       +2     
==========================================
+ Hits          725      744      +19     
+ Misses         19       16       -3     
+ Partials       15       12       -3     

Impacted Files	Coverage Δ
src/dvc_render/vega_templates.py	95.94% <100.00%> (+2.06%)	⬆️
tests/test_vega.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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