Strict types support

[millnut]

With Drupal core moving towards strict types, more info I've added a test pipeline in preparation localgovdrupal/localgov_project#153 which has highlighted some failures which we should fix.

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  Line   modules/contrib/localgov_openreferral/src/Encoder/JsonEncoder.php  
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  37     Call to function in_array() requires parameter #3 to be set.       
  37     Parameter #2 $haystack of function in_array expects array, string  
         given.                                                             
  44     Call to function in_array() requires parameter #3 to be set.       
  44     Parameter #2 $haystack of function in_array expects array, string  
         given.                                                             
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  Line   modules/contrib/localgov_openreferral/src/Event/GenerateEntityMapping.php  
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  120    Call to function in_array() requires parameter #3 to be set.               
  121    Call to function in_array() requires parameter #3 to be set.               
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  Line   modules/contrib/localgov_openreferral/src/Normalizer/ListNormalizer.php  
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  31     Call to function in_array() requires parameter #3 to be set.             
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  Line   modules/contrib/localgov_openreferral/src/Plugin/search_api/processor/AddOrTaxonomyMetadata.php  
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  115    Call to function in_array() requires parameter #3 to be set.                                     
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  Line   modules/contrib/localgov_openreferral/src/SearchApiIndexConfig.php  
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  98     Call to function in_array() requires parameter #3 to be set.        
  140    Call to function array_search() requires parameter #3 to be set.    
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[ekes]

MR merged.