inference: fix inference exception

This PR fixes a `NumberFormatException` spotted during testing ``` java.lang.NumberFormatException: Zero length BigInteger at java.base/java.math.BigInteger.<init>(BigInteger.java:491) at me.serce.solidity.lang.types.SolInteger$Companion.inferType(types.kt:144) at me.serce.solidity.lang.types.InferenceKt.inferExprType(inference.kt:170) at me.serce.solidity.lang.types.InferenceKt._get_type_$lambda$20$lambda$19(inference.kt:259) at com.intellij.psi.util.CachedValuesManager$1.compute(CachedValuesManager.java:173) at com.intellij.psi.impl.PsiCachedValueImpl$Direct.doCompute(PsiCachedValueImpl.kt:77) ```