Fix syntax warning over comparison of literals using is.

[tirkarthi]

Fixes syntax warnings in Python 3.9

find . -iname '*.py' | xargs -P4 -I{} python3.9 -Wall -m py_compile {}  
./PPOCRLabel/PPOCRLabel.py:1035: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if trans_dic["label"] is "" and mode == 'Auto':
./PPOCRLabel/PPOCRLabel.py:1794: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if result[0][0] is not '':
./PPOCRLabel/PPOCRLabel.py:1825: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if result[0][0] is not '':
./PPOCRLabel/PPOCRLabel.py:2011: SyntaxWarning: "is" with a literal. Did you mean "=="?
  assert output is 0, "operate the cmd have some problems ,please check  whether there is a in the lib " \

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