timeseries: fix setting reset causing weird behavior

[stephanwlee]

PR #5030 divided up the default setting from overrides and, in selector,
we have mixed two values in selectors to make a concrete settings value.

When reset, overriden values were set to undefined and caused the
settings to be very wrong.

// This resolves to {a: undefined}
{...{a: 1}, ...{a: undefined}}

Object spread (or assign) does not skip a property whose value is
undefined and causes settingOverride with undefined value to take
precedence over the default value which, in the end, resolves the image
brightness/contrast values to undefined unlike what the types
indicated.

In the end, this is a fault of the type system since the interface is
defined as Partial<> and a value should really not have undefined
value. To illustrate,

interface Foo {
  bar: number;
}

// Legal; `bar` property may not exist
const foo2: Partial<Foo> = {
};

// Legal; `bar` is defined as `number` but here, it is `undefined`
// yet it is legal.
const foo3: Partial<Foo> = {
  bar: undefined
};

TypeScript 4.4 has a way to property guard against undefined value in
a Partial<> type, "exactOptionalPropertyTypes" but TensorBoard app
currently violates the type definition so we will enable the tsconfig in
a subsequent changes.