[wasm] Make WasmMainJSPath optional

[pavelsavara]

There are use-cases when there is no main javaScript file.
For example with NextJs where we don't own application startup.

src\mono\wasm\build\WasmApp.targets

<Error Condition="'$(WasmMainJSPath)' == ''" Text="%24(WasmMainJSPath) property needs to be set" />

src\tasks\WasmAppBuilder\WasmAppBuilder.cs

if (!File.Exists(MainJS))
    throw new LogAsErrorException($"File MainJS='{MainJS}' doesn't exist.");

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Issue Details

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There are use-cases when there is no main javaScript file.
For example with NextJs where we don't own application startup.

src\mono\wasm\build\WasmApp.targets

<Error Condition="'$(WasmMainJSPath)' == ''" Text="%24(WasmMainJSPath) property needs to be set" />

src\tasks\WasmAppBuilder\WasmAppBuilder.cs

if (!string.IsNullOrEmpty(MainJS) && !File.Exists(MainJS))
    throw new LogAsErrorException($"File MainJS='{MainJS}' doesn't exist.");

Author:	pavelsavara
Assignees:	-
Labels:	arch-wasm, area-Build-mono
Milestone:	7.0.0

[pavelsavara]

cc @radical @danroth27

[danroth27]

I hit this as well when looking at using .NET from existing JS apps: #77191