[REVIEW] Remove string_view is_null method

[davidwendt]

Closes #3346
The string_view class has three states: non-empty, empty, and null.
The null-state should not be required since we have a string_scalar class now.
Places where the string_view:is_null() function are being used can be reworked.
These were only found in strings::fill(), strings::concatenate() and strings:join_strings() which all accept string_scalar parameters.

Removing this method also means that string_view has only 2 states now: empty or non-empty. This should solve the requirement of all-empty strings column requiring a 1-byte chars column. This column was needed to distinguish between null and empty when creating a string_view in methods like string_scalar::value() and column_device_view::element<string_view>(). Since this is no longer possible, special logic is not required for handling a null string_view instance.
Therefore, any workaround logic in string.py and strings/utilities.cu to preserve the 1-byte chars column for all-empty strings column is no longer required.

[codecov]

Codecov Report

Merging #4548 into branch-0.14 will decrease coverage by 0.00%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.14    #4548      +/-   ##
===============================================
- Coverage        88.42%   88.42%   -0.01%     
===============================================
  Files               50       50              
  Lines             9686     9681       -5     
===============================================
- Hits              8565     8560       -5     
  Misses            1121     1121              

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/string.py	86.11% <ø> (-0.10%)	⬇️

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[rgsl888prabhu]

Minor changes, other than. LGTM