fix(ngSrc, ngSrcset): only interpolate if all expressions are defined

**BREAKING CHANGE**

If `bar` is `undefined`, before `<img src="foo/{{bar}}.jpg">` yields
`<img src="foo/.jpg">`. With this change, the binding will not set `src`.

If you want the old behavior, you can do this: `<img src="foo/{{bar || ''}}.jpg">`.

The same applies for `srcset` and `href`.

Closes #6984