Return empty SymbolDescriptor for empty symbol

[olafurpg]

This change should fix the following stack trace

Caused by: java.lang.IllegalArgumentException: invalid symbol format
^
	at com.sourcegraph.lsif_semanticdb.SymbolDescriptor$Parser.fail(SymbolDescriptor.java:72)
	at com.sourcegraph.lsif_semanticdb.SymbolDescriptor$Parser.parseDescriptor(SymbolDescriptor.java:143)
	at com.sourcegraph.lsif_semanticdb.SymbolDescriptor$Parser.entryPoint(SymbolDescriptor.java:60)
	at com.sourcegraph.lsif_semanticdb.SymbolDescriptor.parseFromSymbol(SymbolDescriptor.java:20)
	at com.sourcegraph.lsif_semanticdb.SignatureFormatter.symbolDisplayName(SignatureFormatter.java:532)
	at com.sourcegraph.lsif_semanticdb.SignatureFormatter.formatType(SignatureFormatter.java:424)
	at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
	at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
	at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:566)
	at com.sourcegraph.lsif_semanticdb.SignatureFormatter.formatTypeArguments(SignatureFormatter.java:249)
	at com.sourcegraph.lsif_semanticdb.SignatureFormatter.formatType(SignatureFormatter.java:414)
	at com.sourcegraph.lsif_semanticdb.SignatureFormatter.formatMethodSignature(SignatureFormatter.java:164)
	at com.sourcegraph.lsif_semanticdb.SignatureFormatter.formatSymbol(SignatureFormatter.java:38)
	at com.sourcegraph.lsif_semanticdb.LsifSemanticdb.processDocument(LsifSemanticdb.java:140)
	at com.sourcegraph.lsif_semanticdb.LsifSemanticdb.lambda$processPath$3(LsifSemanticdb.java:83)

Reported by @wiwa