Fix a crash for users without a tab theme

[zadjii-msft]

One day into 1.19, and there's a LOT of hits here (76.25% of our ~300 crashes). A crash if the Theme doesn't have a tab member.

Regressed in #15948

Closes MSFT:46714723

[lhecker]

const auto tab = theme ? theme.Tab() : nullptr;
const auto iconStyle = tab ? tab. IconStyle() : IconStyle::Default;

…for the Tab() call deduplication (untested code). Not really an issue tho. Might not even be worth a "nit".

[DHowett]

A crash if the Theme doesn't have a tab member.

We had the same for Window last time.

How do we remove this category of issues?

[zadjii-msft]

How do we remove this category of issues

off the top of the dome:

we change MTSM_THEME_SETTINGS:

terminal/src/cascadia/TerminalSettingsModel/MTSMSettings.h

Lines 138 to 141 in 310814b

	#define MTSM_THEME_SETTINGS(X) \
	X(winrt::Microsoft::Terminal::Settings::Model::WindowTheme, Window, "window", nullptr) \
	X(winrt::Microsoft::Terminal::Settings::Model::TabRowTheme, TabRow, "tabRow", nullptr) \
	X(winrt::Microsoft::Terminal::Settings::Model::TabTheme, Tab, "tab", nullptr)

and instead make them all ctor to {}, so that we instantiate a *Theme object, instead of a null (when none exists)

[lhecker]

FWIW that's not going to be particularly cheap with our current setup, because when the std::optional is empty it returns a new instance every time.

We can make it cheap by changing the implementation to store the WinRT type T as is (without wrapping it) and storing a bool _isDefaulted;. Then we can set the member T and use it as a cache the first time we're asked to return the default value.
(Or something along these lines.)