Description

Fixes issue #5818 where sumBy was returning a string concatenation instead of NaN when encountering non-numeric values.

Changes

• Modified baseSum function to check if values are numbers before addition
• Returns NaN immediately when a non-numeric value is encountered
• Added test cases to verify the fix

Testing

• ✅ Normal numeric sums still work correctly
• ✅ Non-numeric values (strings, null, objects) now return NaN
• ✅ Edge cases like Infinity, -Infinity, and undefined are handled correctly

Example

Before: _.sumBy([{ a: 1 }, { a: 'f' }, { a: null }], 'a') → '1fnull'
After: _.sumBy([{ a: 1 }, { a: 'f' }, { a: null }], 'a') → NaN

Fixes #5818