Support "empty" value in pickers' UX

[dmtrKovalenko]

This PR closes #1613

User facing changes

Breaking changes

• [UI] DatePicker and TimePicker will not render anything if value={null} and the user didn't take any action. (previously it has been rendering defaultHighlight or now)
• Remove defaultHighlight prop

Features

• DatePicker and TimePicker will not render anything if value={null} and the user didn't take any action. (previously it has been rendering defaultHighlight or now)
• Add new toolbarPlaceholder prop that will be displayed instead of day text when value is null. By default "––"

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[cypress]

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Test summary

77 • 0 • 1 • 0

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Run details

Project	material-ui-pickers
Status	Passed
Commit	f360596
Started	May 17, 2020 11:58 AM
Ended	May 17, 2020 12:02 PM
Duration	03:34 💡
OS	Linux Debian - 9.11
Browser	Chrome 78

View run in Cypress Dashboard ➡️

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[codecov]

Codecov Report

Merging #1770 into next will increase coverage by 0.41%.
The diff coverage is 97.36%.

@@            Coverage Diff             @@
##             next    #1770      +/-   ##
==========================================
+ Coverage   90.74%   91.15%   +0.41%     
==========================================
  Files          77       77              
  Lines        2333     2364      +31     
  Branches      439      457      +18     
==========================================
+ Hits         2117     2155      +38     
+ Misses        174      169       -5     
+ Partials       42       40       -2     

Impacted Files	Coverage Δ
lib/src/views/Calendar/CalendarView.tsx	100.00% <ø> (+6.34%)	⬆️
lib/src/TimePicker/TimePickerToolbar.tsx	94.44% <83.33%> (-1.56%)	⬇️
lib/src/DateTimePicker/DateTimePickerToolbar.tsx	95.23% <91.66%> (+4.61%)	⬆️
lib/src/DatePicker/DatePickerToolbar.tsx	96.87% <100.00%> (+0.32%)	⬆️
lib/src/DateRangePicker/DateRangePicker.tsx	100.00% <100.00%> (ø)
lib/src/DateTimePicker/DateTimePickerTabs.tsx	84.84% <100.00%> (ø)
lib/src/Picker/Picker.tsx	95.74% <100.00%> (+0.09%)	⬆️
lib/src/Picker/makePickerWithState.tsx	100.00% <100.00%> (ø)
lib/src/_helpers/date-utils.ts	100.00% <100.00%> (ø)
lib/src/_helpers/time-utils.ts	95.71% <100.00%> (+0.25%)	⬆️
... and 9 more

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[oliviertassinari]

Nice improvement