Fix TypeORM errors for nullable fields

For fields with '| null' in the TS type, the column type needs to be
specified explicitly in column options when using MySQL.

See:
https://github.com/typeorm/typeorm/issues/1358#issuecomment-391736766

backend/src/data/entities/Supporter.ts

@@ -9,6 +9,6 @@ export class Supporter {
   @Column()
   name: string;
 
-  @Column()
+  @Column({ type: String, nullable: true })
   amount: string | null;
 }