Replace null bytes with spaces in session read output

[d3dave]

Terminal cells that have never been written to are represented as \0 in the string returned by async_get_screen_contents() (see [ScreenChar.h](https://github.com/gnachman/iTerm2/blob/9d71745f738f0e44b14160ae2ad08ea3fc441889/sources/ScreenChar.h#L254)). iTerm2 paints them as blank, but consumers that strip or normalize control characters collapse words together.

Before:

After:

[mkusaka]

@d3dave Thanks for the contribution and for the clear reproduction.

After digging into this a bit more, I think this is probably an iTerm2-side issue rather than an it2 issue.

From the iTerm2 side:

• screen_char_t.code == 0 represents a cell that has never been written, not a literal space:
  https://github.com/gnachman/iTerm2/blob/9d71745f738f0e44b14160ae2ad08ea3fc441889/sources/ScreenChar.h#L153-L154
• The LineContents docs in api.proto suggest that uninitialized cells should be represented via code_points_per_cell with num_code_points=0, while text omits them:
  https://github.com/gnachman/iTerm2/blob/9d71745f738f0e44b14160ae2ad08ea3fc441889/proto/api.proto#L1419-L1470
• But the current GetBufferResponse implementation appears to build LineContents.text directly from raw screen chars in a way that can surface mid-line uninitialized cells as \0:
  https://github.com/gnachman/iTerm2/blob/9d71745f738f0e44b14160ae2ad08ea3fc441889/sources/PTYSession.m#L19892-L19950
  https://github.com/gnachman/iTerm2/blob/9d71745f738f0e44b14160ae2ad08ea3fc441889/sources/PTYSession.m#L19990-L20009

Because of that, I’m hesitant to merge a blanket replace('\0', ' ') in it2.

My main concern is that this changes the raw output semantics in a way that could become a regression. If there are cases where a caller actually wants to observe the current \0 representation, or where \0 is intentional data, replacing it unconditionally here would silently erase that distinction.

I’m going to spend a bit more time looking into the iTerm2 side when I get a chance before deciding what the right fix is here.