Language Server: Fix hover crashes on invalid parameters

[henryhchchc]

When textDocument/hover gets an out-of-range request, sourceNode will be a nullptr, which leads to a crash in toRange() (Line 64).

solidity/libsolidity/lsp/DocumentHoverHandler.cpp

Lines 61 to 65 in 7a4ddb2

	auto const [sourceNode, sourceOffset] = m_server.astNodeAndOffsetAtSourceLocation(sourceUnitName, lineColumn);
	MarkdownBuilder markdown;
	auto rangeToHighlight = toRange(sourceNode->location());

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