[Typescript-Angular] add nullable support

Created by: smasala

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Description of the PR

Support v3 spec config nullable so that --strictNullChecks can be used with Typescript

@TiFu (2017/07) @taxpon (2017/07) @sebastianhaas (2017/07) @kenisteward (2017/07) @Vrolijkx (2017/09) @macjohnny (2018/01) @nicokoenig (2018/09) @topce (2018/10) @wing328

modules/openapi-generator/src/main/resources/typescript-angular/modelGeneric.mustache

@@ -5,6 +5,6 @@ export interface {{classname}}{{#allParents}}{{#-first}} extends {{/-first}}{{{.
      * {{{description}}}
      */
     {{/description}}
-    {{#isReadOnly}}readonly {{/isReadOnly}}{{{name}}}{{^required}}?{{/required}}: {{#isEnum}}{{{datatypeWithEnum}}}{{/isEnum}}{{^isEnum}}{{{dataType}}}{{/isEnum}};
+    {{#isReadOnly}}readonly {{/isReadOnly}}{{{name}}}{{^required}}?{{/required}}: {{#isEnum}}{{{datatypeWithEnum}}}{{/isEnum}}{{^isEnum}}{{{dataType}}}{{/isEnum}}{{#isNullable}} | null{{/isNullable}};
 {{/vars}}
 }{{>modelGenericEnums}}

modules/openapi-generator/src/main/resources/typescript-angular/modelTaggedUnion.mustache

@@ -10,7 +10,7 @@ export interface {{classname}} { {{>modelGenericAdditionalProperties}}
     * {{{description}}}
     */
     {{/description}}
-    {{name}}{{^required}}?{{/required}}: {{#discriminatorValue}}'{{discriminatorValue}}'{{/discriminatorValue}}{{^discriminatorValue}}{{#isEnum}}{{{datatypeWithEnum}}}{{/isEnum}}{{^isEnum}}{{{dataType}}}{{/isEnum}}{{/discriminatorValue}};
+    {{name}}{{^required}}?{{/required}}: {{#discriminatorValue}}'{{discriminatorValue}}'{{/discriminatorValue}}{{^discriminatorValue}}{{#isEnum}}{{{datatypeWithEnum}}}{{/isEnum}}{{^isEnum}}{{{dataType}}}{{/isEnum}}{{/discriminatorValue}}{{#isNullable}} | null{{/isNullable}};
 {{/allVars}}
 }
 {{>modelGenericEnums}}