refac: replace linear interpolation with zero-fill for null gaps in metrics time series charts

[djbarnwal]

• Changes bridgeSmallGaps to fill small null gaps with zeros instead of linearly interpolating between adjacent values, so lines visually dip to zero across sparse data rather than connecting directly
• Replaces the en-dash (–) shown on hover for null values with the text "no data", rendered in italic to distinguish it from real values
• Applies the "no data" label consistently across all tooltip variants: simple, time comparison, dimension comparison, and the inline SVG comparison readout

https://linear.app/rilldata/issue/APP-847/metric-point-values-not-displayed-on-hover-after-zooming

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!