Fix expression null pointer exception on expressions with null return type

[TheLimeGlass]

Description

If an expression returns null as it's return type, it throws a NullPointerException at parse time. This pull request makes the error a safe SkriptAPIException.

Error this pull fixes:

[02:29:38] [Server thread/ERROR]: #!#! Stack trace:
[02:29:38] [Server thread/ERROR]: #!#! java.lang.NullPointerException
[02:29:38] [Server thread/ERROR]: #!#!     at java.base/java.lang.Class.isAssignableFrom(Native Method)
[02:29:38] [Server thread/ERROR]: #!#!     at Skript.jar//ch.njol.skript.lang.SkriptParser.parseSingleExpr(SkriptParser.java:540)
[02:29:38] [Server thread/ERROR]: #!#!     at Skript.jar//ch.njol.skript.lang.SkriptParser.parseExpression(SkriptParser.java:752)
[02:29:38] [Server thread/ERROR]: #!#!     at Skript.jar//ch.njol.skript.patterns.TypePatternElement.match(TypePatternElement.java:159)
[02:29:38] [Server thread/ERROR]: #!#!     at Skript.jar//ch.njol.skript.patterns.PatternElement.matchNext(PatternElement.java:54)
[02:29:38] [Server thread/ERROR]: #!#!     at Skript.jar//ch.njol.skript.patterns.LiteralPatternElement.match(LiteralPatternElement.java:65)
[02:29:38] [Server thread/ERROR]: #!#!     at Skript.jar//ch.njol.skript.patterns.SkriptPattern.match(SkriptPattern.java:58)
[02:29:38] [Server thread/ERROR]: #!#!     at Skript.jar//ch.njol.skript.lang.SkriptParser.parse_i(SkriptParser.java:1252)
[02:29:38] [Server thread/ERROR]: #!#!     at Skript.jar//ch.njol.skript.lang.SkriptParser.parse(SkriptParser.java:227)
[02:29:38] [Server thread/ERROR]: #!#!     at Skript.jar//ch.njol.skript.lang.SkriptParser.parse(SkriptParser.java:174)
[02:29:38] [Server thread/ERROR]: #!#!     at Skript.jar//ch.njol.skript.lang.Effect.parse(Effect.java:75)
[02:29:38] [Server thread/ERROR]: #!#!     at Skript.jar//ch.njol.skript.command.Commands.handleEffectCommand(Commands.java:190)
[02:29:38] [Server thread/ERROR]: #!#!     at Skript.jar//ch.njol.skript.command.Commands$2.lambda$onPlayerChat$0(Commands.java:300)
[02:29:38] [Server thread/ERROR]: #!#!     at org.bukkit.craftbukkit.v1_20_R1.scheduler.CraftFuture.run(CraftFuture.java:88)
[02:29:38] [Server thread/ERROR]: #!#!     at org.bukkit.craftbukkit.v1_20_R1.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:480)
[02:29:38] [Server thread/ERROR]: #!#!     at net.minecraft.server.MinecraftServer.b(MinecraftServer.java:1479)
[02:29:38] [Server thread/ERROR]: #!#!     at net.minecraft.server.dedicated.DedicatedServer.b(DedicatedServer.java:446)
[02:29:38] [Server thread/ERROR]: #!#!     at net.minecraft.server.MinecraftServer.a(MinecraftServer.java:1393)
[02:29:38] [Server thread/ERROR]: #!#!     at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:1170)
[02:29:38] [Server thread/ERROR]: #!#!     at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:317)
[02:29:38] [Server thread/ERROR]: #!#!     at java.base/java.lang.Thread.run(Thread.java:833)

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Target Minecraft Versions: any
Requirements: none
Related Issues: none

Addressed