fix : added null guard for activity_date in ActivityDashboard

[tmdeveloper007]

Summary of What Has Been Done:
Added null and validity checks for activity_date before parsing it as a Date. If activity_date is null, undefined, or results in an invalid Date (NaN getTime()), it is filtered out before being added to the activityDates array. This prevents malformed date entries from corrupting the streak counter and heatmap rendering.

Changes Made:

• app/components/dashboard/ActivityDashboard.jsx: Added null guard in the data mapping function. Added isNaN check. Added .filter(Boolean) to remove invalid entries.

Impact it Made:

• Prevents invalid date strings from breaking the streak counter and heatmap display
• Makes the dashboard more resilient to data inconsistencies
• Verified with ESLint: no new errors introduced

Note: Please assign this PR to the tmdeveloper007 account.