fix(mergeDeep): handle `undefined` sources correctly

[ardatan]

Fixes https://github.com/ardatan/graphql-tools/issues/7011

The following inputs and outputs are corrected;

• mergeDeep([{a:2} undefined]) - Any nullish values should be ignored so it should return {a:2}
• mergeDeep([]) - no sources should return undefined
• mergeDeep([undefined]) - no sources should return undefined